Raul is absolutely right. The Pass Labs is special and beautifully made. It will last a lifetime. The 4 Point is one of the best arms for a Koetsu. With the 11" 4 point you might have to add a little weight to the head shell as the Rosewood bodied Koetsus are lighter than the stone bodied units.
Soundsmith sells a great set of graded cartridge screws. With a Hi Fi News Analog Test disc you can get the resonance frequency right down to 8-10 Hz and your bass will be wonderful. The only other thing I can think of that would make an additional sonic improvement would be a low profile MinusK platform under the Firebird. After that I don't think you could do any better regardless of the money you spend. You are right at the point of rapidly diminishing returns.
Soundsmith sells a great set of graded cartridge screws. With a Hi Fi News Analog Test disc you can get the resonance frequency right down to 8-10 Hz and your bass will be wonderful. The only other thing I can think of that would make an additional sonic improvement would be a low profile MinusK platform under the Firebird. After that I don't think you could do any better regardless of the money you spend. You are right at the point of rapidly diminishing returns.