Anyone have a Music First Preamp ?

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.

From George in response to the same question in the Lightspeed thread:

Hi Luka, the Lightspeed will happily slot right into most systems, the only thing that is needed for it to give it's 100% performance is that:
1: The output impedance from your source (cdp, dac, phono stage, etc) should be 200ohms or less output impedance and 1v or more output, which most are.
2: The input impedance of your poweramp should be 47k or more, which most are, 47k being the industry standard.
3: The interconnects from the Lightspeed to the poweramp should be 1.5mts or less and of low capacitance (100pf per foot or less) which most good quality IC's are, this measurement is in the cables specs, if not the supplier/manufacturer should know this.

George says they did a blind test with a panel of listeners and no one could hear a problem until they got down to 30K ohm. I have an LSA on order and a 50K ohm amp, so we'll see.

I've used the LSA with an amp that had a 21k ohm input impedance and it sounded fine, but not optimal as it did with amps whose input impedance specs were higher. From what I've been learning recently, I think in some ways the ratios being quoted for determining best impedance matching are a bit overblown anyway.