What speakers after Snell KII

Thanks, ok let’s say I keep the, for a while.

I got a Eufonika H5 OTL (Miflex KPCU) headphone amplifier with pre outs that I would like to use then. What could be a decent power amp that could work with the Snells? Something used for around $300?

I concur 100%. A sub or two will make a big difference and give you better dynamic range.  I wish I still had my original pair of Snell Type K. 

Why not keep using the KII's? For their size and cost a classic design! I have a pair. For SQ the KII's can compete with any four-figure speakers. You would have to spend four-figures to equal the sound of the KII's! Something like the Quad 57's - of which I have two pairs. A better recommendation is to add a pair of powered sub's to handle the bottom octaves.