Something else to consider is replacing the stock electrolytic 100uF cathode resistor bypass with a film cap (e.g., Solen). Not as big of a difference as the interstage and parafeed couplings caps, but still noticeable (to my ears). But I don't think you can fit both a huge parafeed cap and a huge film bypass cap between the PT and the OPTs, particularly if you have the DC filament upgrade.
I managed it by using 3 x 3.3uF closely matched VCap ODAMs in parallel - they're pretty small, and sound very good (again, to my ears). I also "bypass" them with a 0.1uF Vcap CuTF. It all just barely fits.
Note that a film bypass cap comes stock with the new Monomours.
cheers, Derek