Long distance between preamp and power amp

Dentdog, I don't see any issues with what you've described.  In fact the design of the Leto seems ideally suited to a situation involving a lengthy connection to an unbalanced input.

John Atkinson's measurements of the output impedances of the Leto were even lower than specified, and as I'd expect in the case of transformer coupled outputs did not have much variation as a function of frequency.  That will make the capacitance of the long interconnects a non-issue for any reasonably designed cable. 

Also, I don't know if you are using RCA cables or XLR cables + adapters to mate with the unbalanced inputs of your MC60s, but either way you are in effect running unbalanced interconnections since the load is unbalanced.  However given the transformer coupling and the provision of ground lift switches in the Leto for its unbalanced outputs you presumably are immunized from ground loop effects that can occur in some cases involving long unbalanced interconnections.

Also, actively driven balanced outputs on some equipment (as opposed to transformer coupled outputs) may have problems with XLR-to-RCA adapters, since most such adapters short the signal on XLR pin 3 to ground (XLR pin 1).  But again that won't be a problem with a transformer coupled output, and in some cases involving transformer coupled outputs it may actually be necessary for an adapter to connect those pins together.

Enjoy!  Regards,
-- Al
 

Thanks AL!

Thank goodness we have Al, the voice of reason. Getting rid of that amp just to change the mode of connection would be ridiculous. 

Thank you all for your inputs. I will definitely keep the amplifier, so the bottom line is I will run a good pair of RCA cables inside a shielded sleeve and under my floor - I believe I'll be safe from interferences. 
I work in a theater where we have all sort of professional grade cables (including underwater cables). I'm sure I'll find a good quality cable to perform this task. And as I said, it will be a temporary solution. 

The problem with long RCA interconnects is a loss of high frequencies and definition.

The way to deal with it is run balanced connections to the amp and convert from balanced to single-ended using the Jensen transformers Al suggested earlier. That way works!