I'm not familiar with the Behringer PEQ 2200.
Schematics for most Behringer gear is hard to find, so You are usually restricted to upgrading what you can find out by examining the PCB.
I have upgraded a Behringer MDX2100 Compressor and a DCX 2496 Crossover.
I usually find that most Behringer gear has not got a lot of decoupling electrolytic capacitors on the voltage rails. Usually they rely on lots of ceramic caps.
This is OK electrically but not really the best way to get good audio quality.
I tend to replace any existing electrolytic capacitors, and try to add at least another pair of 100uF 16V capacitors to the voltage rails supplying the audio amplifiers. Usually the rails are 15 -0-15V CHECK!
This seems to give the units a good audio quality boost, especially for older units where the existing components have aged.
The circuitboards are usually surface mount and difficult to rework, so I would not recomend replacing op amps unless you realy know what you are doing. Anyway most hi-performance op amps would require you to ensure that the decoupling capacitors were upgraded to ensure stability.
Any electotytics in the signal path can also be upgraded.
I have had good results with Panasonic FC series capacitors. They are renowned to have very good electrical and audio quality. They are also available in small diameter sizes that are useful for replacing existing capacitors where space is restricted. For instance on the DCX there are 5mm dia signal input caps that I replaced with 6.3mm Panasonics, where the maximum size that would fit between the relays was 7mm.
Check out CPC Farnell for availability of the Panasonic caps.
Elna Silmic capacitors give stunning results but they are massive in comparison so difficult to fit in most existing circuits. I usually buy them on ebay in small quanities.