5751 Triple Mica - really that good?

No experience with 5751s, but Amperex Bugle Boys 1959-1962 LARGE ROUND HALO GETTER presented a huge 3D soundstage, nice clear top end and good detail. Very musical, which I'm not sure the 5751s are.
Check archives for 5751.

My JD-100 has the same mods as yours. I tried a set of the Sylvania trip micas and found they were a little heavy in the bottom for my liking. But this was after I changed the power cord from the stock one to a Virtual Dynamics Power 3 power cord. The new cord added some extra bottom end in itself. I weighed the differences and decided to go with a different tube and keep the cord. I'm currently using a set of TJ Full Music 12ax7's that sound very good top to bottom.

I purchased a Jolida Music Envoy preamp a short time ago which uses 12ax7's and bought a set of Tung-Sol gold pin tubes that produce equally pleasing results. The stock ones were rolled off in the top end.

Your outcome will depend on your system as a whole.

In my experience, the Sylvania Triple Mica 5751 are the best sounding 12AX7 tubes ever produced.

You are right they also cost an exorbitant amount. If you must have the best, you'll pay for them. Outside of that, the current production Shuguang Triple Mica 12AX7B give you most of what the best variants ever provided at about $10/tube. I feel they eclipse many of the vaunted old make 12AX7, and obviously represent off the chart value.

Sylvania 5751 trips are fantastic, used them for years but seldom seen, by me at least, in recent years.

I neglected to say how the Sylvania 5751 black plate trip mica sounded. I agree with the others posting that they sound most excellent.