Cary 200.2/500mb vs. BAT 500 w/BAT-PAK

Bill, Mitch2 and I are pretty much on the same page here. I currently own both the 500.1's and the 500's and intend to keep both. I think these two amps are both very good, but I would agree with Cary's assessment cited above. The 500.1's are a dramatic improvement over the 500's. Noise, resolution, imaging, and tonal neutrality are all significantly better in the 500.1s. I would add a few comments here and encourage you to read my review of the 500.1's, which while give you a feel for how the differences play out in reproduction of music.
The 500's have the most seductive midrange I have ever heard, and a very sweet top end. They are very musical. But the 500.1's are better amps, even if they don't quite have that midrange magic. Mitch2s comments about the lack of control in the bass are on target, but I suspect this lack of control is more noticed in the low base. If your speakers tail off rapidly below 40-45 Hz, this may not be such a big deal for you. I did not really notice an issue with the 500's driving my Maggies, although this may be a contributor to the substantially superior resolution of complex musical passages offered by the 500.1's.
Now then, you didn't ask about the 500.1s but about the 200.2's. I haven't heard them so I can only guess. I would expect that they would share many characteristics with the 500.1's, but you are asking with reference to unspecified current hungry 4 Ohm amps. If you can't stretch your budget to the 500.1's, I'd probably go with the 500's over the 200.2's. They aren't everyone's final answer, but they are very good amps at the going price for used amps.
I haven't heard the Bat-500. I generally like BAT amps, but 9 times out of ten I favor bipolar outputs.

Bill, I have owned the VK-500 with BAT PAK, the Cary 500MB, and currently the Cary SA-500.1 monoblocks, which are essentially two of the SA-200.2 stereo amps, with different input modules.

All of these have pretty good power supplies and the newer Cary amps improve the power supplies even further over the 500MB, since they doubled the heat sink area, included a 1,500kVA low noise transformer (my 500MB transformers displayed mechanical hum while my 500.1s are quiet), and improved capacitance and current, as stated by Cary,

dramatically improved performance and current-handling capacity over our previous models

I suspect there are further improvements of the SA-500.1 monos over the SA-200.2 stereo version (which I have not heard), that go beyond the substantial increase in power since the 500.1s would be a dual differential design with the two halves each driving one phase resulting in the substantial increase in power and better noise cancellation.

The other basic design difference between the newer Cary amps compared to the 500MBs are the use of a larger number of smaller capacitors located closer to the output devices, rather than the fewer large capacitors used in the 500MB. They have increased the overall capacitance.

A distinguishing design difference between the Cary amps and the BAT is that the BAT uses mosfet devices in both input and driver stages and the Cary amps use bipolar output devices. You can do searches as to the characteristics of these devices but some would say mosfets have a softer presentation and bipolars have better detail and slam - as a gross generalization.

Regarding how the amps sound, in my system, the BAT did indeed have a sort of softer pleasingly warm midrange at the expense of some definition and HF extension. The bass was quite solid but a bit "woolly." The 500MBs had a sweet sounding HF, and a smooth, rich midrange that many like, but the bass suffered from a lack of control and was somewhat plump as a result. The 500.1s improve on the 500MBs dramatically in the bass, which is as good as I have heard, and also improve in the areas of clarity/resolution, soundstage, and dynamics, while having a much lower noise floor than I experienced with the 500MB.

In summary, they are all ok but I would tire of the BAT over time and would choose the 500MBs over the BAT, and certainly the 500.1s over the 500MBs, as they are pretty much better in every way.

I haven't heard the Cary but I think you would really like the BAT.

I should also add that this would replace a Rowland Model 5. I had posted a thread a few days ago asking about a Rowland Model 8 or possibly a pr. of 6's. As they don't come up for sale very often, I'm looking into the above as "substitutes" for now.

I'm still looking for the sweetness in the mids and highs the the M-5 has but am looking for better low frequency definition. The M-5 actually has good macrodynamics but is not quite as good on microdynamics. Don't want any lean or thin sounding gear.