I'd love to know where you've seen an 18kHz rolloff from the empeg; it's a LOT flatter than that to beyond 20k. The home outputs had worse rolloff on early mk2s, but this was fixed on the mk2a (requires removing 2 caps). The car outputs weren't affected.

If you have bad channel separation (on an early mk2, sernrs <about 500) then you can improve this by changing one component - this was the wrong part being fitted by the manufacturer. It doesn't affect the mk2a's. Channel separation should be >~75dB on mk2as.

Hugo