Regismc,
Never heard the Duelund Alexandria's. I have heard the Duelund VSF, and the Clarity MR. Both are great, both can't go wrong. And the only comment on Mixing was directed toward trying to combine another type of cap either bypassing a duelund or in a circuit like a crossover using more than one type or several positions of capacitors, then your better off with all Duelund 100% or all Clarity MR 100%, not a mix of the 2.
As for your amps I am sure both are great, but Clarity MR's I have used in an all Tube preamp with stellar results, and I have used a Duelund VSF in a tube PHONO amp with also zero compromise results.
The VSF may be slightly more natural, the Clarity MR might have a little more detail which doubles as "Airy". Depends on the tubes, if they are very soft or warm it might be a nice contrast using the Clarity MR's as you have already had first hand experience. Also Size consideration physically comes very important in play. If a Duelund or Clarity MR' depending on the Value you need is just too big to fit.
If one of these caps has a smaller body for the comparable value (Alexandria vs. MR) than you might be better off with the physically smaller cap depending on the space you have available as both are winners. The VSF is a bigger flat body almost guaranteed to take more space and may not fit at all, then again depending on the setup might fit better with a big flat cap opposed to a big round body cap.