Just to put myself right...
I recall someone commenting that the OPA1612 was very good, but performance degraded at higher frequencies.
So I looked at the TI datasheet, and sure enough the SINAD got worse above 1kHz, but then I had never seen any THD vs Freq charts for any other OpAmps.
ASR is a goldmine of information, and I found this today:
This is a study of whether changing op-amps in a DAC makes a measurable difference. Taking a lead form "rolling" tubes in tube products, a trend has started in replacing op-amps -- sometimes with much difficulty -- to improve performance of solid state products like DACs. A few months ago I...
www.audiosciencereview.com
Yes, the OPA1612 SINAD gets worse above 1kHz, but then so does everything else (and this is tested in a DAC, not a power amp):
The only niggle is that the bottleneck may be elsewhere.