A little guidance needed...

Right, here's the situation...

Current setup:

Amp - Pioneer VSX C10

Fronts/Centre - Mordaunt Short Alumni 

Sub/Rears - Pioneer S-V40 Speakers

I'm shortly going to replace the amp with the Onkyo TXNR609 or similar Denon models around that price bracket and also likely going with the BK Monolith as a quality sub upgrade.

I'm reluctant to do away with the Mordaunt Shorts at the front at the current time (I was given the centre at Xmas to upgrade from the Pioneer) but I will be without rears if I can give/flog the Pioneer gear as a package as i've still got all the speakers for that.

So do I just pick-up some rears to retain the MS Alumni unison at the front, or would it be an idea to move the MS Front left/right to the rears and pick up some better speakers for Fronts to go along with the new amp/sub?

If I were to do that, how much would I realistically need to spend (i'd probably look to get floorstanding) to match the Onkyo? And would the best option be to get Mordaunt Short Aviano/Mezzo if i'm retaining the centre speaker for now, over say a nice pair of Monitor Audios?

I also have some John Lewis vouchers that could cover something upto around £400 but i've only seen the Q Acoustics 2050 Tower Speakers there, which I assume wouldn't be enough of an upgrade to be worth it, especially if complimenting a MS centre.

Any advice appreciated..