Anyone with knowledge on if there are any minor/significant differences between the Mcintosh mc2500 and mc2600?
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!
I snagged a C2500 just after the C2600 was introduced.  I believe the main performance difference is the C2600 has a newer, and "better" ESS DAC chipset and it has the new headphone amplifier built in.
As I don't use the built in DAC and hardly ever listen to headphones, the C2500 seemed like a better choice for me.
Thanks for the replies but I'm talking about the MC2500 amplifier and MC2600 amplifier both by McIntosh 
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:  
I have one 2500 and was thinking of getting another. Then came across a 2600. On another note what are some thoughts of either of these vs the Theta Citidel mono blocks?
effischer, i see that the 2500 and the 2600 are enclosed. i guess that's why they both have a fan. the 602 is open.
@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.
I agree with Rhodes on that.  They had dozens of them.  I think Phil Lesh had a separate output for each string on his bass, and each output had a dedicated MC2300.  A fine amp, but, at at around 125 lbs., a bit on the heavy side.
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Well here's the latest I ended up with a set of Theta Citadel amps due to the MC 2600 being in California from a private seller and the Citadel's being being 2hr. Drive at a brick/mortar store that I've done business with in the past and most importantly I was able to hear them first. 
I'm now asking any one familiar with the Citadel's to let me know what preamp's mate well with them? 
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only reason i posted it at 350 watts per chanell is because on ebay there's a pair for sale and it says it's 350 per chanell.
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I own both. 

The MC2600 is much faster than the MC2500.

The MC2600 is a modern power amplifier, whereas the MC2500 is an old generation amplifier. I enjoy both of them very much.