Tell us more about those JBL subwoofers; power requirement, etc.
The Onkyo M-504 stereo amp is a vintage beauty.
If using System 1 or 2 it will drive 4 Ohms speakers.
It might be a good idea to put a fan on top, those flat types that are efficient and quiet; over the warmest spot. Anyway 250 watts into 4 Ohm loads should be avail in the Onkyo M-504 stereo amp.
But like I said, I would check the power requirement of those big JBL subs first.
And that M-504 is simply gorgeous, one of the best looking and proud of it.
You know because you own it, and you also know how it looks inside as well.
The rating is 1 kVA for the transformers (power consumption), so it's not a kids toy here, it has serious juice (current) and enough microfarads total to sing with aplomb.
Sure, it's not a $100,000 amp, it's a $2,000 one (slightly below, MSRP) ... circa mid-80s (1987).