C2600 recommendations

I suggest you look into some Audio Research Reference 6 or Reference 10 linestage analog preamps (no built-in DAC, no built-in phonostage, strictly linestage preamps) or any Audio Research linestage preamps if you can’t afford the Ref 6 or Ref 10 preamps. Any Audio Research preamps, especially their Reference series preamps, will be a big step up sonically from any McIntosh preamps. But all ARC (Audio Research) linestage preamps have no built-in DAC or EQ.

So, you will have to purchase an external DAC or streamer/DAC or CD/SACD player. All high end and ultra high end or uber expensive preamps are pure analog (no built-in DAC) anyway.

If you’re going digital audio front end sources I suggest you look into getting a really good digital front end source components to connect to an analog linestage preamp. There are countless options out there when it comes to searching for either a standalone DAC or a streamer/DAC or streamer + DAC or CD/SACD player that will be superior sounding to the built-in DACs in any McIntosh preamps or most preamps.

DAC or digital circuitry is noisy and will be better off separating the digital audio stages from an analog linestage preamp stages if your budget allows.

@caphill
Audio Research is not a step up from McIntosh.  Have you heard the ARC equipment compared to the McIntosh?

@stereo5  

ARC is indeed quite a bit of step up sonically from McIntosh, especially the ARC Reference series gears, but cost way more than McIntosh. Especially when it comes to preamps (both linestage & phonostage) and DAC & CD players. No comparison but the ARC cost way more than the McIntosh. ARC are more musical, more resolved and better overall and music had more presence with ARC.
The ARC Ref DAC & Ref cd player retail for around $25k each whereas McIntosh D100 DAC retails for only around $2500 to $3k and the McIntosh MCD600 CD/SACD player (McIntosh's flagship CD player) retails for only $6500. 
They are simply not in the same league.
People who can't afford ARC (Audio Research) would probably end up buying McIntosh. 

I have compared the two-stacked ARC Ref 10 linestage analog preamp ($30k) to the two-stacked McIntosh C1100 analog preamp ($15k) driving a pair of Sonus Faber Aida ll speakers. The ARC Ref 10 preamp is paired with 2 pairs of ARC Ref 750 SE vacuum tube monoblock amps ($70k/pair & $140k for both pairs) bi-amping the Sonus Faber Aida ll speakers. And the McIntosh C1100 preamp is paired with 2 pairs of McIntosh MC kw1.25 monoblock amps ($25k/pair & $50k for both pairs) bi-amping the Sonus Faber Aida ll speakers. No comparison. The ARC Reference series stacks way outperform the McIntosh stacks.
For comparison purposes we used the same digital front end source components and analog front end source components. For digital front end source components we had the DCS Vivaldi full four stacks (Vivaldi masterclock, Vivaldi upsampler, Vivaldi DAC, Vivaldi CD/SACD transport) with a Naim Core Reference music server used for storage. These DCS Vivaldi full stacks are cost-no-object design digital front end source components and the full stacks retail for $125k. 
For analog front end source components we used a cost-no-object design Clearaudio Statement v2 turntable ($220k) with a Statement TT-1 tonearm ($20k), Statement Goldfinger cartridge ($16k), external discrete class A linear PSU for the turntable, and the D'Agostino Momentum phonostage pre ($28k). So both digital and analog front end source components are of high bar and should give the downstream components & speakers justice. 

Performance wise the ARC Reference series stacks are true reference quality and cost-no-object design gears whereas McIntosh higher end series stacks are good but not reference quality nor cost-no-object design gears. 

We also compared both the ARC Reference series stacks (Ref 10 + Ref 750 SE) & the McIntosh stacks (C1100 + MC kw1.25) against my own Naim Statement NAC S1 linestage analog preamp + Naim.Statement NAP S1 monoblock amps driving my Magico M6 speakers. These Naim Statement combo are cost-no-object design gears and retail for $270k total ($90k for the linestage analog preamp & $180k for the monoblock amps). For analog front end source components we used my Clearaudio Statement v2 turntable with a Statement TT1 tonearm, Statement Goldfinger cartridge and my D'Agostino Momentum phonostage pre. 
As for digital front end we used my MSB Select ll DAC with a flagship Femto 33 clock, two mono powerbases (two mono power supply units) one feeding power to its digital side of it and the other feeds power to its analog audio stage. These MSB Select ll DAC stacks are also statement piece and cost-no-object design gears. And we used my Naim Core Reference music server as storage. 
My Naim Statement preamp/power amps indeed did crush both ARC Reference series stacks & McIntosh stacks. The Naim Statement pieces are on another level. Myself personally coming from these Naim Statement gears and have been using these Naim Statement preamp/amps combo as my reference pieces and do judge other components from various different manufacturers based on the performances of my Naim Statement preamp/monoblock amps and haven't found anything at any price points that perform better than these Naim Statement pieces. However, having said that, I can easily live with those ARC Ref 10 linestage preamp + ARC Ref 750 SE monoblock amps eventhough it is a downgrade sonically from what I was accustomed to. But I don't think I can live and put up with those McIntosh stacks. 
I'm not trying to ditch McIntosh but I have such high expectations and those ARC Reference series gears mentioned above could easily live up to my expectations although they don't perform on the same level as my Naim Statement gears. 

I recently put a C2600 into my system and am very happy with it. 
I would take it over an ARC preamp any day. 

@gmercer 

You should have a listen to the ARC Reference 10 or Reference 6 linestage preamps. These aren't in the same league as the McIntosh C2600 or any McIntosh preamps. Especially the ARC Ref 10 which is a true reference quality linestage preamp and is cost-no-object design. The ARC Ref 10 is a two-stacked preamp with PSU encased in a separate compartment and retails for $30k, which costs more than 4 times as much as the C2600, which retails for only $7k. And the ARC Ref 10 or even the Ref 6 will be a huge step up sonically from the C2600 or any McIntosh preamps including McIntosh's flagship C1100, which is also a two-stacked preamp. 

However, the ARC Ref 10 or all ARC preamps don't have built-in DAC or EQ like the C2600, so that makes C2600 huge bargain in comparison to any ARC linestage preamps. FYI, all true high end and ultra high end reference quality linestage preamps are pure analog and don't have built-in DAC or EQ. DAC or any digital circuitry is noisy and introduces noise to the preamp's analog circuitry and compromises sonic purity & sound quality. DAC and other digital circuitry need to be external and in a separate compartment. 

The ARC Ref 10 ($30k), D'Agostino Momentum linestage pre ($35k), Naim Statement NAC S1 linestage preamp ($90k), Boulder linestage preamps which are also cost-no-object design and cost up to $70k, Ayre Reference linestage pre ($27k), Naim NAC 522 preamp with its external 555 PS DR power supply unit ($32k) and any other cost-no-object or price-no-object design preamps from CH Precision, Tidal, MBL, Soulution, Burmeister, Chord Electronics, VAC, VTL, T&A, Lamm are pure analog and have no built-in DAC or EQ. At this level or those who can afford these ultra high end or cost-no-object design preamps would also want cost-no-object design external digital front end source components eg standalone DAC, streamer/DAC, CD/SACD player that will sound way way better than the built-in DAC in the C2600. 

Some entry to mid-level affordable preamps such as the C2600 and some other McIntosh preamps could often times have built-in DAC and even EQ but the sound quality can never compare with those high end or ultra high end cost-no-object design pure analog preamps such as the ARC Ref 10, Ref 6, D'Agostino Momentum, Naim Statement NAC S1, Naim NAC 552 with its 555 PS DR external psu, Ayre Reference, Boulder, Burmeister, Tidal, MBL, CH Precision, Soulution, etc, etc.