I think I'm having a similar problem with my Spider IV 75w amp. My volume is extremely low. I have to crank it higher than I ever would during normal play just to hear anything at all. Heres what happened...

It was working just fine about an hour ago. I turned it off for a while and started messing around on the Spider IV Edit in "offline" mode. I then connected my laptop to my FBV Express MKII through the usb cable and turned my amp on (I'm pretty sure I did it in that order). I double clicked on one of the settings I downloaded from Spider Online Tones, expecting it to automatically transfer to my amp like usual, but it didn't. So I clicked the "send selection" button on the editor and it went through. Up until this point, I hadn't played a single note since I turned the amp off an hour ago. Bam! No volume... From the speaker or through direct out. I tried everything I could think of besides a factory reset to fix it. Switching channels, turning it on and off, disconnecting the power cable, disconnecting my laptop from my FBV pedal, disconnecting the FBV pedal from the amp entirely, checked all the volume, boost, and drive settings, used different guitars, different amp cables... nothing seemed to work.

Not sure if this has anything to do with it but I just remembered that at some point during the hour the amp was turned off, I turned it on without connecting it to the guitar or connecting the FBV to my laptop. Just the a naked amp basically. And out of curiousity (wishing it had a secret dual tone mode I never knew about), started pushing two channel buttons at once (A & B, B & C, etc).

I'm not even sure if any of that is the cause of my problem, but I figured I'd give a bit of detail anyway.

EDIT: Factory reset fixed. Still not sure how it happened.

I suggest downloading the Spider IV Edit software and saving your presets to your computer before reseting it.