BAT vk-3ix and Parasound Halo A21 match?

With an output of 1K on the BAT preamp you will be fine. I used a BAT VK-31se with the Parasound JC-1s and Thiel CS7.2s and got good sound.

How is the bass

It sounds like you've already read John Atkinson's measurements of the preamp in Stereophile, in which he says that the balanced output impedance "ranged from a very high 14k ohms at 20Hz to 1850 ohms in the midband and above."

It can be calculated that into 66K that will result in a rolloff at 20 Hz in the rough vicinity of 0.2 db, along with some extremely small effects on phase. Both of those effects figure to be imperceptible, and to be swamped by room effects and probably also by the deep bass rolloff of the speaker.

As I have no experience with the preamp, though, I can't comment on its bass performance apart from the impedance matching issue.

Also, given the relatively high output impedance at high frequencies, if the interconnect cable length is particularly long (say 15 feet or more) I would make a point of using low capacitance cables. Otherwise a very slight but possibly audible rolloff of the upper treble could result.

Regards,
-- Al

Yes i did read the stereophile article and did read his concerns.

On the other hand the Thiels only go down to 32 hz so .2'at 20 hz seems ssomewhat irrelevant. I use Purist audio cables which have low capacitance and are only 14' in length anyway.

I went to BAT's website and they say minimum input amp impedance on the amp should be 10k, though I'm sure that is just a guideline.

I guess I'll know for sure once I get it in and hook it up.

I've used my 3ix with a Bryston 3B-SST, which has an input impedance of 10k/leg balanced. It is a few dB down at 20 Hz (measured with a voltmeter at the amp outputs), but nothing disastrous. The Parasound should be fine.