I've never heard those particular speakers and i'm sure they sound nice, if you're perfectly content with the sound then you should most definitely stick with them. For your speakers, either amp would be a waste of money in my opinion, and you can find something that will drive them just fine (and will sound the same) for much cheaper.
Just for reference, I have a pair of harder to drive 6 ohms, 87.5 dB bookshelves powered by a HK 3485 integrated/receiver (2x120 watts) and I can effortlessly push the speakers to ear bleeding levels. Doubling the power rating only provides 3dB more headroom, and those E60s are easy to drive speakers so even just a receiver like an HK would be more than sufficient for really loud volumes.
If you are dead-set on buying either the A21 or XPA2, for your particular case, i'd easily choose the XPA2 due to price. The A21 is a nicer amp designed by a brilliant engineer, but it's completely unnecessary for those speakers, especially considering the price difference. In a real-world listening application, I am nearly certain you wouldn't be able to tell the difference between both amps in an ABX test.