What after Aleph 3, need more juice?

You can get another Aleph 3 for biamp or bridging.
I always prefere bi-amp.
Another option is Plinius SA100 used.

Try a Pass Labs-X-250 for around 3K or an X-350 for around 4k. It will make those puppies sing.

Unfortunately, you cannot bridge the Aleph 3's (per info from Pass Labs).

I crossed that bridge three years ago, when my a 3 simply couldn't push the 4 ohm woofers in my new Parsifal Encores.
A pair of Aleph 2 monos continues to provide incredible satisfaction, especially well served by an Aleph P. I'm hanging them from floor joists under each speaker, with 8' ribbons of SPM flowing back up through the baseboards. Quite stealthy, which was VERY important to Ellen....
Pair of the Aleph 2 continue to sell under $3k. Such a deal!

I've a similar system, WP5.1 and Aleph 3 in a medium sized room so I know why you like this combination. I have a number of other amps as well, when I want more power I switch to a BAT VK-200, it really integrates well with the WP's. You should give this a try, they cost about 1650 used (maybe less now that a new model is coming out from BAT), 100 WPC into 8, 200 into 4. And you won't miss the heat buildup that you are used to from the Aleph, the BAT runs very hot. The only thing is that they have balanced only inputs and you need an adaptor if you don't have a balanced pre.