Join Date: Mar 2006
-10 is generally what you work with in guitar audio, +4 is for "professional" equipment. Although, your FX loop might just be a piece of shit, but i thought the FX loops on the more expensive marshalls were pretty good..
Read this (I didn't write it, just copy-paste)
Q&A Questions and Answers general sound questions
GENERAL SOUND QUESTIONS
What does '+4dBm' and '-10dBV' mean on equipment? Is it important?
These values apply to the expected input and output levels. The levels of +4dBm and -10dBV are the 'nominal' levels, not the maximum or minimum, but used to differentiate between equipment for 'consumer' applications which generally have lower level, high inpedance inputs and outputs (the -10dBV level) and equipment for 'professional' applications, which have higher level low impedance inputs and outputs (the +4dBm level).
Both are voltage levels, and as you will see from the table below that the +4dBm equipment will produce up to 4x the level of the -10dBV equipment
level in dB level in Volts (RMS)
If you connect equipment with different nominal levels, then yu may experience distortion, particularly if a high impedance output is connected to a low impedance input, as the output may not be able to deliver the necessary voltage. Generally, low impedance outputs will connect to high impedance inputs, although you will have to be careful with the level output.
Now, if you're doing it the wrong way, you might be experiencing
#1 Gain decrease, loss of high end
#2 Gain increase
If you have both on +4 the return buffer will be severely mismatched with the output of your FX.. and you'll lose gain and treble. Plus, you're killing the input on what you have connected to send :P
BTW, some FX processors can switch between -10 and +4 too.. If you can, low impedance is generally better.
Proud member of the "$20000 worth of pro gear but can't play worth shit" squad