Best way to reduce compression ratio?

I would think so too, is there a particular rpm or throttle setting/load it pings at? In any case, just because its only 91 v 93, it may actually be a totally different blend of fuel with equivalent octane rating, as Dave alluded to. Octane boost is OK as long as its only to keep it from pinging until you get to a better gas station, but it doesn't raise octane really, it alters the properties of the fuel to resist detonation by chemistry...MMT.

Example: my Mustang ran 9.2:1 compression with a Vortech supercharger...best gas around me was 91 octane, and I could get 12 psi boost with iron GT-40 heads and NO detonation anywhere...but boost wasn't in until above about 2500rpm.

I'd probably back off a couple degrees ignition lead or at least alter the curve to where there's less timing under low rpm/increased load and swap to a colder plug, if that doesn't work, time to play with cam timing or changing heads.

You mentioned running J heads, do they still have the big chamber relief? That's surely doing you no favors for quench.