I just started building my HT system, and for a receiver I bought a Pioneer Vsx-521-k. I have a pair of Sansui Sp-2500's right now hooked up to it and I will be adding more speakers as I am able to afford good ones.
The speakers sound good, but it seems lacking on the bass, which I find odd since they have a 12" woofer in them.
I also am considering pulling the trigger on some Polk RM6750's, maybe it is time to do that and get rid of these?
check the speaker setup menu in the receiver and make sure they are set to "large", or whatever Pioneer calls full frequency speakers that can handle the lower bass frequencies. You will need to check the crossover frequencies to confirm if they are set lower and not cutting off low bass to the Sansui's.
PS: I have a set of Sansui speakers with the 15" woofers (mid 1970's era) hooked up in a den for a stereo system......still love the sound those beasts produce !!! SP-7500 models....I think....haven't looked at the name plates in a number of years.