I really appreciate everyone's thoughtful replies.
I realize, as a couple of you pointed out, that I lose bit-perfect output if I use the JRiver volume control. But, to my layman's mind, I think that is a relatively benign loss compared to other dsp functions. I understand that JRiver employs 64 bit processing using internal volume, so losing a couple of bits of that is a modest loss. I rarely reduce digital volume more than 12 dB (two bits, as I understand it.)
I've attached an interesting article about digital vs. analogue volume adjustment. It appeared in PS Audio's free "Copper" webzine, which is an interesting read. Richard Murison, the author of the attachment and proprietor of BitPerfect software, concludes that digital volume adjustment yields better audible results than analogue controls. Murison seems to know what he is talking about. But he is in the business, so I employ the usual cautions.
Thanks, again for your prompt replies.
PS--DJLegba: I checked my ASIO driver and, unfortunately, it does not allow adjustment of volume in individual channels.