Hey-hey!!!,
before I started building amps which wanted lower gain linestages I was happy with the 5687. I found a general trend that higher voltage combined with lower current made it sound a bit crisper. Perhaps in a thinner sort of way. It is a fairly subtle change, but fairly easy to spot. Shoot for 80% of B+ and 3 mA. Keep in mind that an L-C supply is going to react differently at loads below critical current. Proceed with caution and a multi-meter, no?
cheers,
Douglas