ARC Ref 75's as monoblocks?

These are differential amplifiers. I don’t think they can be run as monoblocks.
Bridging amplifiers isn't usually a good idea even when an amplifier is suited for it.

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(((ARC Ref 75 amps as monoblocks? )))Joe
 Running one channel of each Ref 75 would allow the power supply to isolate, have less saturation on demanding peaks. A significant advantage with faster recovery.  
 

If you were to run just one channel of each Ref 75 it would be prudent, IMO, to put load resistors on the output of the unused channel.

Tube amps having output transformers should of course not be operated unloaded when they are processing a signal.  But in this scenario, where one channel is not processing a signal, consider what would happen if a tube developed a short or some other problem that would cause that channel to output a "signal" while unloaded, and with no means for the user to become aware that anything is amiss.

I agree with the comments about bridging.  Paralleling the channels is another possibility, but I certainly wouldn't do that without an ok from ARC, and I can envision ways in which a cloud of smoke might result.  For example, if a small signal tube in one of the paralleled channels were to fail, the other channel would be applying a large signal to the output of that channel, while that channel would be trying to force its output to zero volts.  Not a good situation.

Regards,
-- Al
  

In general, I am not a fan of bridging amplifiers. It's sort of like a parlor trick and, yes, sometimes the result is ok. But if you want a monoblock amplifier, I think the best thing to do is buy one rather than kludge a stereo amp.