There tends to be a bit of false conventional wisdom about reproducing string quartet music because of the fact that we are dealing with four instruments instead of a full orchestra. String quartets can have significant dynamic range and the fundmental on a Cello is at 65hz. So while it is 20hz above a double bass, it's still well down in the bass range.
Apart from the PS Audio, any of those other amps in a big room are going to need a pretty sensitive/efficient speaker. The DeVore 96 probably fits the bill best.
I would also look at Harbeths but do not rule out listening to one of the bigger models like the HL5 Plus XD.
If you really want a stand mount speaker and are prepared to compromise on bass extension, the original Sonus Faber Guarneri is worth seeking out.