question re: poor bass response

Apologize in advance if this is a naive question. Unless they are played very loud, I have been a bit disappointed in the bass response from my Sonus Faber Concertos driven by NAD C370 integrated; source: Arcam cd92. However, at reasonable sound levels, the volume control on the amp is positioned only about 8:30 - 9:00. Is it possible that I'm underpowering the speakers when they're played at lower volume (sensitivity of speakers is 87dB, 1W, 1m). Would bass response improve if I switched the jumpers to the variable pre-outs and reduced the gain to the power amp, such that more power would be required to achieve a given sound level?
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?
Newbee, the room is probably an audiophile's nightmare. Overall dimensions are ~40x18x9 but a partial wall separates the listening area (24x18x9) from the adjacent "room". The speakers face the length of the room, sit about 10' apart and 3' in front of a set of built in shelves/bookcases.

Hammergjh, speakers are on Sonus stands with spikes, so isolation shouldn't be an issue. I'm curious, what amp did your Krell replace and why do you think the Krell gives you better bass levels?
The Krell replaced an Odyssey Stratos. Krell amps are known for their prodigious base response, and now I know why. The bass became palpable at all listening levels but very balanced, not overly pronounced. The difference between the amps was striking. I'm not knocking the Odyssey, but I never felt the Odyssey handled the woofers that well and the Krell handles them like a champ.
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.
Yes you may be underamping. But your main problem is likely speaker and seating placement/positioning, and their relation! If your speakers or seats are not coupling well in parts of frequency spectrum, you WILL get a weak bass response! Also, from a "height perspective", if your ceiling is 8', the standard Sonus stands at 30+ inches are too high in my experience! The speakers do better aimed up slightly on 24-26" stands at best!
Still, you need to put the speakers where you sit,play music, and move around near the room, with your head down where the speakers go!. Then you move around till the bass sounds right!...repeat for each speaker.This is the easy quick way to "earball" the response for each speaker. If you don't get your seats and speakers set up where they couple best for "flat frequencey response", you'll get poor overall response in the bass. You can also measure when you're done with test tones and a sound level meter to make sure. IF it measures reasonably flat from the bottom of the bass to the upper bass, you'll have dynamic, full, fast, accurate sounding bass!
Learn to set up speakers in a room, and you'll be rewarded. Also, if a large large room, don't expect deep room filling bass, as the woofers can't fill the room. Good luck
i would get a different amp and go from there.
I would reposition the spkrs, of course. The amp may be a tad light for the job -- but should do for now (unless you're bent on investing).