Upgrade amp or pre

My current set up is:
Yamaha CXA5000
Mac MC205
SF Cremona Ms

I'm thinking of either adding the MC452 or upgrading the pre to an Anthem Dv2 or the Krell foundation. I would say 60/40 for home theater. I can't upgrade both, has to be one or the other, where would I get the biggest gain, amp or pre? Thanks in advance.
I have "upgraded" amps only to be left wondering how it is that different brands can sound so remarkably similar; later asking, why did I spend that money??

However, every pre I've purchased has made a substantial alteration to the sound of my system (not always for the better). In my opinion, the Pre is the crucial, vital component that brings everything together. A good pre can make a mediocre system sound great.
For the price of a MC452 (15,000) You could upgrade BOTH the Preamp and the Amp. Not sure why your focused on there most expensive two channel amp? You could get a MC352 and a C2200 and have a bunch of cash left over. IMO. Regards.
surround sound pre amps because they have integrated DAC's suffer faster technology obsolesence... doubt the Macintosh amp technology has changed much in a decade. Be sure to identify a specific problem you are trying to solve in a new pre else you may get a better pre in an area that doesn't enrich your listening enjoyment.
Thanks guys for the input. I'm not trying to solve any particular area, just want to improve the sound. Feel like my speakers can sound better.