OK, I think I just saved myself some money (for the time being at least).
By plugging the front-firing ports in the JBLs I got rid of the boominess that was irritating me and then by moving them slightly back into the corners I got some of that upper-bass/lower-mid warmth back that I was missing and assumed would need new speakers for.
The booming I'd been having problems with had forced me to bring them out from the corners, therefore losing some of the warmth too. Now I've plugged the ports, I can get it back without being drowned in BOOM! I love the way the Bryston keeps a grip on that 12" bass driver. The JBL L65s have a sensitivity of 89dB/1W/1m so I think the 2B-SST has plenty of power to drive them at the levels I listen at, although I've no idea about the JBLs' impedance curve - any idea where I can find this online? Come to think of it, I'd love to find some detailed measurements for the L65 Jubals, a la Stereophile-type measurements. All I can find is this:
http://manuals.harman.com/JBL/HOM/Technical%20Sheet/L65A%20Jubal%20ts.pdfAnyway, one happy camper here (for the time being...

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