I've got a digitech RP-300A, a lovely little multi-effects pedal that I enjoy using, and a Fender Frontman 25W amp.
This is the problem: there seems to be no possible way to connect the box to the amp without it being obscenely loud. So far I've *fixed* this problem by turning down the volume of the pedal down to around 3 (out of 99!). The problem is with that is that the pedal defaults it's volume to 34, enough to blow my poor little amp. I'm afraid somebody (such as my brother) will turn both of them on without realizing what they're doing and ruin my gear!
I'm running the pedal to the amp using the headphone output on the pedal to the aux input on the amp.
I've also tried running the left output of the pedal to the main input of the amp using a normal guitar cable...this almost works, but the volume on the amp has to be kept around .5, or else it gets dangerously loud.
From what I can tell, this problem is the result of the pedal's preamp amplifying the signal, then sending it to the amp, which also amplifies it, resulting in a really loud signal.
Is this what's causing my problems? If so, how do I fix it?

Firstly, the connection to your Fender amp should be from the left output of the pedal.  The stereo headphone out is just for headphones.
Secondly, you should not have the "Drive" switch selected on your amp.
Lastly, choose a clean effect on your pedal and have it at the default volume (of 34)

With the above set up, at low amp volumes does the guitar signal sound distorted?
If it isn't distorted, then there's no problem at all with the pedal or the input stage of the amp.  This means that cranking it up won't damage the amp or speaker.
You may simply be playing somewhere where even 10W would be inappropriately noisy; but there is another possibility that your amp is not functioning correctly.
Does the same thing occur when a different amp is used?  If so, it's the pedal - if not, it's likely to be your Frontman.

If the sound is distorted even at low volumes, then the input stage of the amp may be faulty or there may be a problem with the output stage of your pedal.  Again, try it in another amp.  If the same problem exists (distorted input even at low volumes) then it's the pedal, if not, it's your Fender.

OK, so I've not exactly solved the problem - but hopefully given you food for thought.

For a while I tried using the left channel of the output of the pedal, but when that gets plugged into the amp, I have to have the volume set less than one, otherwise it'll blow the speaker. Seriously, all it would take is a small bump, and everybody's ears would be ringing and I'd be out looking for a new amp
Thanks for trying to help!

But have you tried it in another amp to see where the problem lays?

Also, what's the guitar volume like without the pedal into your amp.

There are so many things to try - unless there's a fault with either the pedal or the amp, there should be no problem plugging in and turning it up!

I think I'll try using a different amp. My guess is that it isn't just one component that's to blame, but the combination of the two is the problem. I can run the digitech box to my computer with the input volume normal and the pedal volume normal...
As far as the guitar volume is concerned, I like to keep it cranked up (so it stays very sensitive and I don't have to screw with the preset noisegates). From my experience, the explosive sounds happen regardless of the guitar volume, unless it's all the way down or withing that range.

I didn't make myself clear..... I mean that, if you plug the guitar straight into your amp does the problem go away?  (thus showing that it's clearly a pedal to amp issue)

Double check whether the output impedance of the pedal is within range for the input impedance of your Frontman.  There's a small chance you may have already damaged your amp if you weren't.

Impedance is measured in Ohms and has a symbol like: Ω.

Awesome! I'll check that out...
One more question: what do I do to fix that? Is there something I can run it through to make the output impedance match on both? I  really don't want to get rid of any of my equipment!

I assume you don't have soldering and circuitry skills to build a pre-amp...

Run it into the same input on the amp as you would your guitar normally.

IF there's an input impedance mis-match (and it really is a big IF) then you could run it through a DI box.  They're inexpensive, and are able to cut the signals down to an acceptable level for most equipment. As a rule of thumb, use an active DI for passive signals and a passive DI for active signals (such as the one from the pedal).  Pretty often an active DI is fine for all applications though.

Ah, yeah.

Big if, yes, but an if nonetheless.

yup try just plugging in your guitar to your amp and see what happens ( I'm pretty sure you have done this already).  Check all the volume levels through out the chain of effects ( I looked at one of these and there are several places you can adjust volume( effects levels ect)).  your running out of earphone output so I have no idea what the ohms would be....usually your regular outputs on these effects units are fairly well matched to amplifier inputs(hard to sell units that won't match up to anything).  I don't know this amp but does it have preamp gain and master gain controls?  If It's like my amp 5 is ear bleeding loud (master volume).  my amp runs at  preamp 8 and master volume at practice on 1 1/2  if I'm using amp distortion. If I'm not using amp distortion then  about 4 on the preamp and 2 sometimes 3 on the master volume (still pretty loud). 

Most multi effects units you have to adjust volume levels in the preamp, distortion boxes, and all other effects  your using to get the right sound. 

without being there and it's near impossible to tell what it is though.

Thanks for all of your help!
Yes, it does work correctly when I plug my guitar directly into the amp, so the pedal is to blame.
I couldn't find any information on the output impedance of the digitech box, but the amp is 8 ohms.
When hooking the digitech pedal up to my computer, the signal isn't absurdly loud, so I'm almost certain that the problem is because the pedal amplifies the signal, and then the amp amplifies it even more, resulting in the really high volume. Soon I'll get a chance to plug it into a friend's amp, and hopefully it'll work then.

you probably have this already but if you don't  go ......here and read through this .
http://img3.musiciansfriend.com/dbase/pdf/man/m_150179.pdf

I have the same pedal and won't work at all with either of my rigs unless I plug it in the send Efx and by doing that I bypass all volume and tone knobs.The pedal it self becomes the master as I slave the amp.It works for some settings but not all sounds are good so I have to tweak them here and there and i get what I need.So in general it pretty good but is a little messy on the heavy parts and bit twangy on the cleans but again tweak it for better results.