Anyone have a Music First Preamp ?

Al the contact and pricing information can be found here:

http://members.optusnet.com.au/~georgehifi/

Google it. You will likely get the LONG DIY thread on all things LSA. You need to buy direct from George in Australia. Should be <$500 with a Walwart to power the LEDs.

>significantly north of 50K ohms, at least 100K ohms to be conservative

really? rut-roh

I suspect 50kohms would work fine, now if you are looking at 10-20kohm you might be pushing it, though some listeners claim it can also work for them. George from LSA does specify >50kohm - tubes in other words (for the most part. But, that's why Nelson Pass built the B1 - has to drive his SS amps:)

50K ohms input Z may work fine for you, but it's on the low end of the range. YMMV, but personally, I'd aim for higher than the minimum.