The phono preamp is to inversely replicate the RIAA curve for low frequency responce of all RIAA equalized records (which most are). If it is not doing a good job of faithfully replenishing the curve of it's original values, the playback will suffer.
Several other questions come to mind and some are simple (or silly) but need to be asked.
1) Do you have the cartridge wired correctly? Red wire is Right positive, green wire is Right negative, White is left Positive, blue is left negative. If you have 2 of these wires reversed, you would be out of phase and the left woofer would be going inward, while the right woofer is going outward, cancelling your bass response.
2. Are you tracking at the proper VTF (vertical tracking force) your cartridge will have a recommended weight (in grams) and hopefully you have taken care of this part during your set-up.
3. Have you (or can you) adjust your VTA (vertical tracking angle)? The arm with the stylus on the record should be parallel to the record surface. If the pivot end is too high, the recording can sound brite and shrilly. If the pivot end is too low, it will sound clouded and muddy.
That's enough to get you going for awhile.
Wayner