tough choices

wow, that is about  £62,000,uk pounds to spend, KEF makes speakers that are tri-amped, some in early thousands, don't know why just do, you will always get a better sound with three amps, per speaker, but just two better amps will sound better for the same money, 
2 amps at £30,000, 
6 amps at £10,000,
the two amp system will be far better, in every case, but you still have to buy with a demo,  

Saying they “need” to be tri-amped is extraordinary. Maybe that is what is required to get the absolute most out of the but that is a choice.
I would look seriously at AVM monos. If you really want six, the Ovation MA 6.3s are awesome amps and will just be to the warm side of neutral, tonally similar to ARC IMO. They are $23K/ pair here in the US. At current exchange rate, that would give you six at about $100k CAD.

I am an AVM dealer here in the states but am not allowed to sell in Canada and I will not trans-ship. If you are interested, I would be happy to reach out and find you a dealer in Canada. PM me and let me know what part of Canada you are in and I will pass the info along.
Getting 6 good sounding mono-blocks at $20k to $25k USD per pair should be quite feasible. AVM is one very good option.  I think you will struggle to find something from Constellation/ Soulution/ Boulder/ D’Agostino/ Burmeister at that price though and IMO, AVM is very close in terms of sound quality. It is a strong value all things considered.  
The other brand to look at is Pass Labs. You can easily heat a portion of your house with these since they are Class A and 100w each but six XA100.8s will be a little less expensive and I think will run about $90K CAD.
The Pass X600.8 are more comparable to the AVM at 600w of class A/B power each and would run a bit more than AVM at what should be about $110K CAD.
I am not a Pass dealer but finding one won’t be that difficult.

IMHO
Trade in The Mono Blocks for the 80% they offered.
Trade in The SF Aida for 80% as well. I don't see why the dealer wouldn't do it as all Models come from The McIntosh Group. In fact AR left the group a few months ago and the dealer is still offering the trade in. That would give you a substantial amount to invest in products that I believe would give you more bang for the buck. - 

For speakers - in general look to increase your frequency response, especially since you have the 4 RELs put them to work and really open up and enhance your sound stage. 
Check out - even if you don't go this way its good to know the comps.

Lansche Audio 5.2 in price range/ 7.2 just over - plasma tweeter gets up to 150kHz - requires some maintenance.
Verity Audio Lohengrin IIS - the ribbon tweeters gets up to 60kHz.
The SF Aida tops out at 38kHz

For Amps I agree with moving to solid state mono blocks especially paired with the tube preamp.
I also like the Ayre R twenty.  MX is the mono block model.
dartZeel is another company to check out as well.
I also found the Boulder lacking a bit and it was paired with Wilson Audio...

Get your dealer working for you, let them know that you are not in the market for a tri-amped set up. Let them come with solutions that meet your needs.

Best of Luck