Hmm. Not quite following what you're saying here. The Aegir uses a single-ended pair of inputs for stereo, and one balanced input for monoblock mode. Both the Freya S and Freya+ have both single-ended and balanced outputs (the back panel has the same inputs and outputs). To get your Freya S to your one Aegir you are using single-ended outputs, which Freya+ also has. So why does this require one to use two Aegirs with Freya+?
Hmm, let's see if I can clarify this. Technical stuff is not my forte, but here goes.
Prior to the Nexus topology (and predecessor Pivot Point in Jotenheim) the way Schiit moved from balanced input to SE ouput was via summing the signals. But there is a loss of sound quality in doing so. It has been called negligible but in my own experience using Mjolnir 2 it is real: using Senn 650 headphones the balanced output from MJ2 was distinctly better than using the summing to SE out, with either the MJ2 or my headDesk hp-amp (different cables of course).
Nexus changes that, it gives one the same sound quality whether the output is balanced or SE. (To me that is an incredible technical advance!) Having experienced the loss, I wanted to avoid it. Freya S has the Nexus topology but Freya + does not, it gets the SE output by summing. ed: Oops, No, See *. Wanting to avoid that loss is why I went with Freya S. (Also while I like tubes on the input side I am not as enamored with them on the output side.)
Note e.g. that BCowen uses Freya + with two Aegirs. Perhaps he needs the extra headroom, or perhaps he has learned that Freya + sounds better balanced out than SE out. (ed: Oops, I misunderstood his post... I took him too seriously.)
Now does Freya + SE out (even with the summing) sound better than Freya S SE out? That I don't know. But I do know (I think, I haven't actually made the cf myself) that to get the full benefit of Freya + one would need two Aegirs.
Hopefully that helps.
*ed: There is an error here. The conversion from balanced to SE with Freya + (and likely the original Freya) does not use summing and apparently has no sacrifice in sonic quality, only in less power. I am not a technical person in audio and missed that change in topology. Sorry for the misunderstanding.
Noah