Yes, the FET’s a 2SK170GR.
From what I can tell, it’s the same circuit as the 603s (on a different board, of course).
For improvements, replace the cap on the bottom with a 820-1000pF COG ceramic, polyester, or polystyrene – try all three and see which you like the most. Replace C3 and C9 with a 0.22uf metalized film (Panasonic “V” series) or polyester. A larger value like 0.33uF might be better if it will fit. A larger cap in these places will pass a little more low-end.
Lastly, I’d try removing C1 and C2. This is just personal preference, though. You may not “hear” a difference, but I just feel like it’s unnecessary clutter. If you start picking up radio stations in Moscow or something, I’d put some 0.01uF ceramic COGs or some type of polys in their place.
Q4, L1 and L2 form part of a DC-DC converter that boost the phantom power voltage to a higher value. This give the mic more headroom and can improve the sound. Other MXLs like the 2003/V93, and 1006/57/63 use this method. I’ve heard that SP does this with their mics too.
(edited to correct info)
