You don't mention what speakers or preamp you'll be using, or how much power you require, so only a general recommendation can be made.
First, is the Belles 350A Reference (or 150A Reference). While I don't consider it "dark", I do consider it to be on the warm side of neutral...certainly with forgiving highs. The 350A Reference will run you about $2000 used. The 150A Reference about $1500 used.
Another option would be a McCormack DNA-1 or a DNA-0.5
Naim has been said to be dark. It runs on class A B
Out of production, Conrad Johnson MF2250 would be my first choice. The "A" version will be a little more expensive, but still under $1500.00, and are easy to find. The MF2250 also has two bigger brothers for consideration.
The ARC is one fine amp.. they' usually list for 1600-1900, but don't come around very often. Both retain their value well, plus you'll always get good support from both companies.
Thanks for the recommendations. My speakers are Sehring 703s (86 dB sensitivity). The preamp is an old Aesthetix Calypso. My current amp is a McIntosh MC275, which I generally like, but it doesn't offer up very convincing bass with my somewhat bass-shy speakers, heats up my living room to an uncomfortable extent, and seems to have raised my monthly electric bill quite a bit. So I am thinking about downsizing to a less expensive solid state amp that might sound similar to the B&K I used to own, but is newer, more reliable, and will be serviceable if the need for repair arises.
All of the amps I mentioned have an input impedance of 100k Ohms, and will match well with the Calypso preamp's 1k Ohm output impedance. They will all drive your Sehring 703s speakers without any problem.
The Belles 150A Reference is current. The 350A Reference, although apparently out of production, can be serviced by Dave Belles.
The McCormack amps are older models. However, if you can pick up a DNA-1 (or DNA-2) modified by SMc Audio (Steve McCormack), it can be worked on be Steve if it ever requires servicing. He will not service McCormack amps that have not been upgraded by him. Both of these amps are large and heavy, however.
I owned Monarchy SM70 Pro monoblocks, and they were definitely on the warm side for SS amps. They'd have similar power as your MC275, and so would probably drive your Sehrings to similar levels. They're Class A and run hot, but not blazing. I loved them with my Von Schweikert VR-1s, but they were a little noisy with my Druids. The new Mk IIs are supposed to be quieter than the originals I had.
I second the Conrad Johnson for a warm SS amp. You might also want to try an older Mac, although people have argued that their SS amps are not all that warm, I think they are but to each his own. In any event you should get the bass balance you seek.
Primare is known for soft tonality - the I21 budget amp being quite dark, the I30 more neutral and of higher transparency but still with a shared house sound.
Similarly, the Exposure 2010 and 3010 both have a non-fatiguing presentation.
'Warm' and 'Dark' as used in audio are different to me.
Maybe a Sim Audio moon W- 3 or 5? Maybe one of there new amps?
The treble has detail without being etched and emphasized so could call them a touch dark. I wouldn't call them warm though in terms of lush midrange but you have a tubed pre.
I'm running an Aesthetix Calypso with Sim Audio W-6 monos and the combo is great with fantastic bass.
Thanks for the recommendations. My speakers are Sehring 703s (86 dB sensitivity). The preamp is an old Aesthetix Calypso. My current amp is a McIntosh MC275, which I generally like, but it doesn't offer up very convincing bass with my somewhat bass-shy speakers...
The Mcintosh 275MkIV is a powerful amp that shouldn't have any problems driving s speaker with a sensitivity of 86dB. The bass quality is very high when compared to other SS amps like the Plinius SA-100Mk3 that I've owned. I am not familiar with your speakers, and there could be a possibility the perceived shy bass is due to the speakers rather than the amp. Food for thought.
By the way, the Plinius amps are dark sounding and quite similar in presentation to the Mac but lose out in finesse and bass depth. I have compared the MC275MkIV and Plinius SA-100Mk3 in my system side by side with an ARC LS16 line stage.
Plinius and MBL are on the warm side.
Audio Research 100.2 is class D amplifier. See review in TAS Nov 2008 issue