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.

In the good old days preamps had a "Loudness"control which automatically boosted the bass as volume was lowered. It was based on the Fletcher-Munson curve mentioned above. Audiophile purists stamped out Loudness controls, along with Tone controls on most preamps, and they also frown on equalizers. If it really is a low volume problem, a subwoofer will not solve it. You need bass boost, over more than subwoofer frequencies, and an equalizer may be the only way to get it.

I also was plagued by a poor bass response at low and mid volume levels with my B&W CDM9 NT floorstanding speakers. I improved upon this dramatically by uncoupling the speakers from the floor via soundcare spikes. I received a welcome increase in bass at all levels. Then I added a Krell integrated to my set up. Once again another improvement in bass levels.

Are your speakers on spikes or appropriate stands? Also, is your amp up to the task?