Mcintosh

DSD = Direct Stream Digital; sigma delta modulation @ 1 bit, 2.8224 MHz sampling.  It is the SACD standard.  The C2500 has a pulse code modulation-only DAC that goes up to 32 bit, 192 kHz sampling.  PCM is the foundation of the CD Red Book standard (16 bit, 44.1 kHz).  The DAC in the C2600 has both PCM and DSD.

Regardless, the real magic of the C2300/2500/2600 series is the analog phono section.  One of the best out there.

Hope this helps!

Thanks for the clarification and sorry for the oversight!  Mc model codes can be tricky in the amps/preamps section if one isn't paying close attention.

Regardless, the MC2500 and MC2600 are generally considered the two most "classic" McIntosh solid state power amps ever introduced.  The MC2500 was the foundation of the "Wall of Sound" during the Grateful Dead stadium tours back in the day.  They used a truck-full of them at every show and never blew one up.  One of McIntosh's claims to fame today:  They have never had a catastrophic amplifier failure reported.

Differences?  500 WPC versus 600 WPC.  Identical architecture internally.  Almost identical cosmetics.  Bullet-proof reliability.  They made more MC2500 units than MC2600, so the 2500 series is more readily available.  Similar pricing on the used market.

I've not heard the MC2600, but the MC2500 set the standard by which all other SS amps were measured at the time and I found it to be detailed, effortless and quiet.  I can't help but have high confidence that the MC2600 would be the same.  

If it was my money, I'd probably try for the 2600, just because it's the rarer piece and I haven't heard it.  If I didn't like it, turning it around on a 2500 would merely be a matter of time; losing money on either unit is nearly impossible.

Good luck & happy listening!

@g_nakamoto:  The MC602 commenced production ~2000 and is still available, while the MC2600 ran from 1990 to 1995.  MC602 is quad balanced, was tested to a far wider frequency response range and has a better S/N number. 

MC2600 has better specifications than the MC2500, but both are very similar.  Note that they are fan-cooled, while the MC602 is convection cooled.  Additional details on both can be had from Roger Russell's site here:  http://www.roger-russell.com/amplif2.htm.  

@2channeljunkie I can't offer any input on the Theta amps; never ran across any.  I vastly preferred the MC2500 to SAE and Phase Linear at the time, but found the Threshold Stasis to be a bit more enjoyable with the turntable rig I was using (Magnepan arm, DV23 Ruby, Sota Sapphire).  Couldn't afford either, so stuck with my Hafler DH220 until I could swing my Belles Model One.  

@g_nakamoto Not necessarily; the 2500/2600 were designed as caged, rack-mount units and McIntosh decided to produce a form that could be used both by consumers and professionals.  That's why the fans are speed selectable and can be set to thermostatic control.  602 is strictly a consumer design; glass faceplates don't travel well in Anvil cases.