Neo Zuko
a) That sounds like an awesome mini system you're building
b) But, too much talk about stereos and not enough about headphones!
c) Still, I would go with getting a sub before getting a second power amp to run in mono. The full-range factor brought by a sub will make a far bigger difference than the clarity/power advantages of dual mono amps
d) Why would you even need two mono amps if the first one is already giving you 75 watts? Especially running bookshelf speakers like the LS50s, which are the crux of a mini system and not something to blow up a concert hall. Kef themselves recommend power input of 25w - 100w, so a single stereo amp at 75w puts you near the top of their recommended range...
d.5) If your goal has less to do with power, and more to do with going dual mono, I'm sure you could maintain your budget but put an emphasis on fewer watts with higher quality per watt - not that we know the quality of watts in the Timekeeper yet, since it still isn't released... But if your goal is, in fact, power and loudness sufficient for a larger room, then you'd do better off with other speakers than the LS50s: As amazing as they are supposed to be, even the best bookshelf speakers ever made would be undermatched in a very large space.
My .02
