First off, has it been mentioned, we can assume coolant, right?
Dunno about H softness if you are already under rpm with 50HZ input.
As noted above, i do fine (no pun intended) with J. You are not grinding hard steel, the wheel does not need to be that soft as the soft metal will pull grit out. Running lower surface speed already lowers the effective hardness of the wheel. A new full size J wheel running at 2400 is probably already acting like an I or H at 2900 +/-rpm unless the table is running too slowly. It acts softer as size decreases (SFM goes down) too. FWIW, I hardness used to be a common hardness here, though if not true in GB won't help you.
(Edited) _ was thinking my 10" wheel machine is 2850rpm, that yours was similar. Looked up specs for yours, 2570 rpm? so you are not that much lower. Nonetheless, a wheel at 2400 will already act a little softer than when run closer to max rating)
Is your current wheel loading up? or is it dulling? Or is it staying open but losing the dress?
Is it possible your wheel head is floating?
I have spend a few years of cumulative hours in front of surface grinders but my only "pro" time was nights in a friends tool building/sharpening shop 20 years ago. The rest of my experience is self absorbed.
including a lot of advice on here. With that in mind, I tend to grind surfaces for finish with a "significant" downfeed and maybe .005" step over per table reversal. Wheel dress/sharpness lasts a very long time because most of the work is occurring at the edge of the wheel. Heat into the part stays low, too. For heavy stock removal, I'll go down as deep as .030 with a couple to .005" step over per reversal.
Per wheels - Michigan Buck made me aware of how much sharper Radiac brand wheels seem in general, at least compared with Norton. I mostly buy surplus NOS wheels, so use both. Recently found a crate of old Waltham 46J wheels. Would not necessarily recommend them, but for me the cheapness trumps spending for fancier, for most work. Esp since i shape a lot of wheels. Easier to think "might as well shape it, only $5, instead of "wow, this has been a nice wheel but it cost $30, do i shape it for a short job and put on the peg after or not?"
smt