Adcom GFP 555II Preamp Bass Lacking

I recently acquired an Adcom GFP 555II for 50 bucks. When I first tried it I noticed that it sounded very flat, had virtually no bass at all (lab output) and a slight amount of bass with the contour on. I replaced all the old capacitors with ELNA caps. It sounds much better now, however it still has a weak bass output. Any ideas on what could be wrong or is this what I should expect from Adcom? I'm running an Adcom GFA 5200 Mosfet power amp as well. Running four 10" bass drivers but the bass is just not there. Thanks for any help, -Keith