Sorry for my rather 'limited' response I was in a bit of a hurry. Zanash is correct, what I really meant is that the LK1 would be a long way down my second hand 'to purchase' list. If you would like second hand equipment of that age (and general type) I would strongly recommend Naim which sounded better and can still be serviced (Linn will nor repair an LK1 and they are known to go wrong). The rest of your question is a bit vague I'm afraid. I have listened in depth to the 640p and I prefer my 'vintage' Naim gear (some do not). There is a great deal of Naim versus 640p discussion on PFM at present (and in the past). The 640p isn't a 'modern' pre-amp though - in fact, it's not a pre-amp, just a phono stage, you need a pre-amp as well.
I would buy 'quality' vintage - however, not the Linn amps.