If I had to allocate $2K & $8K then I would pay closer attention to the preamp. I've gone through 5 preamps in the past year all of which were in a similar price category (~$500 to $1500 used) and they all had their own distinct character, strengths and weaknesses. And these differences were more pronounced than the differences between power amps that I have tried over the years. This of course assumes that your amp is at least sufficient to drive your speakers in the first place. Not that there's a magic formula but I would rather split the $10K more evenly, perhaps $6K preamp, and $4K power amp. (And again, only if the $4K power amp will be sufficient for whatever speakers you are driving).
