I'll take a shot at answering this, but as of yet I have not tried my Orcas with anything but my class A PP 6v6 amp so I can't comment on the s.e.x. or the 2a3, though my plan is to use mine with the stereomour when I get it done.
I have done most of my listening so far with just the Orcas bythemselves and no subs and found them quite acceptable for a lot of musics, but for the full spectrum and a fuller sound, even one sub will do it -- you'll be surprised how full this sounds at low listening levels. My impression is that without the subs you get a "usable response to around 50 hz, give or take -- which means you can hear it but it is not as full as you might like -- I really can't predict that for you, you'd have to listen for yourself to know if that willl work for you. With the sub there is just more body there, more substance, a fuller presentation, though it's not like you just added a wall of 15" subs -- it just completes the musical picture very nicely. I don't even really like to think of these as subs, but as bass extenders/ bass modules for the Orcas.
You should give the system 400 hours of playtime to make up your mind though as things will fill in, even out, and paint a more consistent sonic picture with time.
As for my setup, I have two buf subs on order for these Orcas because the room is fairly large (open floor plan and the inter main floor of our home) and because I want to use them as stands for the most compact, uncluttered look I can get.
Given your moniker, I'm pretty sure you will love these speakers with either of your amps, but keep an open mind about the sub, and I'd say that it would be easy to use just one for a great system.
I'm already thinking about a second pair of Orcas for my office desk/computer speakers -- to be used with my original s.e.x. 2.0 and my HRT MS II+ dac
Hope this helps,
Jim
Jim Rebman -- recovering audiophile
Equitech balanced power; uRendu, USB processor -> Musette DAC -> 5670 tube buffer -> Finale Audio F138 FFX -> Cain and Cain Abbys near-field).
s.e.x. 2.1 under construction. Want list: Stereomour II
All ICs homemade (speaker and power next)