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I have a small system in my office that consists of leftover odds and ends: old NAD electronics, an Integra music server, and Linn Tukan speakers. Being in an office environment, the music plays at low levels and, not surprisingly, generally sounds hollow and washed out. Is there some way to get a more dynamic, "punchier" sound at low volume levels? Would it help to switch to active speakers? Are there any particular affordable speaker/amp combinations that sound good at low levels?

Thanks in advance for your thoughts and help.
From experience, Dynaudios get really good sound at low volume levels if they are powered properly. IE you are using a discrete amplifier that is designed for 4ohm output. I think many premium speakers that are properly powered by discrete amplification and using good cables should give you better results. Another possibility is adding a small powered subwoofer to add some depth to your soundstage. A little extra bass can add tons of depth to the sound, even at really low volume levels.

Affordable is a relative term. You can pickup a pair of used Dynaudio 42's here on the gon for $299 right now plus shipping. If that is out of your budget, you can pickup Jamo speakers from Vann's right now dirt cheap. Those Linn speakers should be fine, but it sounds like they aren't getting enough juice, so you might try upgrading the amp driving them and maybe trying a speaker cable upgrade.

Cambridge audio has a new micro system dubbed "the one" that offers 30 watts of discrete power and an integrated iPod dock ($600) and the Sonata AR30 is a 2.1 receiver and has 40 watts of power ($500). They also sell exceptional full size receivers all with discrete multi output amplification. I personally think they make some of the best receivers out there for the money. I auditioned dozens of receivers before choosing the Azur 640r and it sounded as good or better than most and some were hundreds or thousands more.
I think as a general guideline highly efficient and easy to drive speakers are going to sound less compressed and more dynamic at lower volumes. That means a 4 ohm floorstander is probably not going to fit the bill. Monitor speakers are inherently more efficient than florstanders but you might want to consider some designs that are higher than 4 ohms.

As for a recommendation I'd suggest a monitor speaker from Green Mountain Audio. Not only are they generally efficient and easy to drive but the phase and time coherence will really aid in hearing dynamics and detail at low volume.
Preamps offering tone controls or loudness contours address the no guts at low level phenomenon.
You may want to consider powered NHT M00s and their PVC passive volume control. I'm listening to a pair playing back digital files from a MusicVault Server ouput thru a HDT MusicStreamer+ DAC. You have the server; the Music Streamer+ DAC and M00s would cost you under well $500 used. Compact, neutral, detailed, punchy and if you want to take them to the next level you can add the NHT powered S00 sub. Otherwise the loudness contour suggested by Rockvirgo is a good idea.
After thinking about this for a little bit, and if cost is a major issue, you might consider klipsch speakers. From what I have heard and know about their construction, they are one of the most efficient speaker currently made and would definitely drive well at low volumes. Some people don't like them due to inherent sonic qualities from their design, but as far as efficiency at low volumes on a cost versus benefit basis, they would do well. Monitors would be more effective at low volumes, but the Dynaudio monitors are horrifically expensive due to their studio design and build quality. Anyway, I just thought I would mention Klipsch as an option since a really nice pair can be purchased for little $$ and the current setup should drive them admirably at low levels. If you add a small powered sub with the klipschs that should help balance some of the highs that will be a little pronounced from the klipsch design.
Thanks again for your responses. You've all provided some good ideas for me to think about and implement. I like the tonal qualities of the Linn speakers, so perhaps the first thing I'll try is adding a small powered sub.