question re: poor bass response

Low bass level when your are playing music at low levels is a function of the ear/brain interconnect. The bass needs to be elevated at low levels to match the "apparent" output of the mids and highs. (The phenom is referred to as the Fletcher Munson Curve.) Thats why receivers etc have bass tone controls - so you can apply your own compensation.

Apart from that, it could be your speakers have inadequate capacity to energize the bass in your room - but we don't know about your room, as you didn't tell us.

One solution is to buy a seperate sub woofer which you can use to supplement the bass, using it at a slightly higher level than the main system, or you can buy an equalizer and install it in your tape loop and use it only when you want the additional bass from your speakers.

Changing the gain to your preamp will do nothing to alter the problem.

I suspect that your speakers are too small to properly provide low bass in a room as large as yours, let alone full sounding bass at low volume. Even with full range floor standers you would need some means of elevating the bass level due to the F-M curve discussed above to get good bass at low levels. Despite comments to the contrary, it is possible to utilize a sub woofer - it all depends on the crossover and amp which you use on the sub woofer. My objective would be to have a sub which operated independly of the main speakers and didn't start to roll off the high frequencies until 100 hz - that way you could turn it off for high levels of playback. If you don't want to invest in a sub woofer, equalizers or tone controls are your only answer, short of buy new speakers and amp/preamp.