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Well, the LL1627A is very good universal transformer with phenomenal flexibility (properly done - with section not with tabs) to load the output stage. It might be a good choice for a full-range amp at 200mA, though I would still prefer to have a transformer gaped for 300mA – so you would need a larger core and they do not do it. I do suggest to by one LL1627A as you would need to know how to load your 515c, your S2 and how Milq might sound all together.

For LF transformer you need nothing fancy but juts with a lot of core mass. In my case Jack from ElectraPrint did very well, it is else very inexpensive but there is a catch in there – you do not know what kind ration you would need to damp your 515c driver properly (do not forget –Milq is no feedback amplifier). You might use the LL1627A to investigate your bass channels’ needs – that what I would do. Alternatively you might try to do what I am trying to do now with my 1Ohm (amorphous?) bass transformer – to make the secondary interchangeable. It has some complexity as the coils would not fill up the core as they use to and the person who would wind the thing would need to make the necessary corrections in order to hit the target impedance.

I have a left over bass transformer from my project but it would not be useful to you primary because my transformer dies at 800Hz and it would be too low for you. If I was you I would go for top roll of at minimum 2000Hz as 550hz is very high crossover point.

For above 550hz you might use the same LL1627A if you wish, for MF the 200-250mA gap will be fine. Yes it would be reasonable to desire less inductance in the transformer, mean less turns but honestly I personally did not detect that it very much affects sound, at least in context of the S2 bandwidth.
Well, Mark, there are a number of manufactures out there who made amorphous transformers, I did not try them, I am sure you will found a lot of people out there who had more exposure then I did. I kind of had an easy ride with LL1627A.

For a single stage amp you would need a transformer with ~40mA and the inductance enough to care 2-3 octaves under your crossover point. Here is fragment of email that I was sending to transformer manufactures when I was searching my MF transformer.

1) Amorphous 'C' cores (there is different type of amorphous material, I
would need with higher permeability)
2) Ratio 15:1
3) Minimal amount of turns on primary, juts enough to built up inductance to support high-pass roll off at 500Hz.
4) Gaped for 40mA
5) The most minimalist phase shift above 10Khz.
6) 300V on primary.
7) Lowers DCR on primary
8) <5w carrying power.
9) Inclosing and appearance of the transformers are irrelevant
10) The secondary sections (>7 sections)go out (with marked polarity) allowing secondary remapping.

Still, if I was you I would not think about the transformer for a single-stage amp or about anything else. Get a single LL1627A if the equivalent amorphous, it will cost you $350. Build a single mono Melquiades around this transformer, powering it from your bench PS but with the Milq biasing schematics. It would take you one evening to do as Milq is very simple amp. If you found the full-range result worth you attention then you might think further. While you do so you will see how you bass channel react to 8H of the LL1627A’ inductance.

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