Best interconnect for Spectral amp, RCA or XLR??

must it be MIT cables

I'd go with Transaparent before MIT which I used to sell.Many models have those blocks and what they did was roll for the frequency exrtremes.To the un-initated this sounds "better" because you have "more" but in fact they have less information.Don't thik this the way to go.Otherwise I wouldn't bew mulliing over selling my CVT interconnects that sit inmcloset.Plus I have problems with end caps breaking and given the price of MIT that should not happen.
Chazzbo

It has been quite a few years since I had my all Spectral/MIT system, but I found an XLR interconnect b/t preamp and power amp(s) to be a significant upgrade over SE IC's. I had the identical preamp you are using with an older DMA-200 power amp.

Spectral electronics are designed to be single ended, and the company and dealers always recommend RCA. However if you're using a long cable run between the pre and mono's, i.e. 20-25', you should use XLR for its noise cancelling capability. The amps need the RF filtration provided in some networked cables to avoid any potential for destructive oscillation. While MIT is Spectral's preference, they used to, and probably still, recommend Transparent as well. If you can upgrade later to the DMC30SL series over the DMC20ii, you'll find it noticeably better with your amps and speakers.