Try it and see.

> *power trannies, 220V/18V (5K:32Ohm), at around 3VA, as the total circuit power is 1,08W*

Think MUCH bigger. Distortion will be significant at 1/3rd of rated AC voltage and frequency (power transformers work with HUGE distortion; only the power company cares). 50Hz is a reasonable bass-power limit for audio, but you should not ask your 220V winding for more than about 70V at 50Hz. 70V^2/5K is 1 Watt, so you are at the AC limit for low THD at 50Hz.

The DC limit is nominally zero. For torroids the real limit is very-very small. For EI stampings, most constructions that have an air-gap no matter how small, you may get away with DC current about 10% of rated AC current. You want 12mA DC, you probaby want more than 120mA AC rating, 0.12A*220V= 26VA rated core!

Fortunately 25VA cores are not much more expensive than smaller cores.

Also: it is hard to put numbers on it, but a little core wants to have low inductance. In power use, this means large idle current. But for small transformers, who cares if 3 Watts flows to the load and 5VA sloshes through the winding? On big transformers, idle current can overheat the windings, and big cores tend to give more inductance, so a bigger core will give more bass.

Response will be good from 50Hz or below (depending on plate resistance and NFB) up until the leakage inductance gets in the way. I did some tests once. A 2-bobbin winding, 115/230V on one leg, 12V on the other leg, gave terrible response, falling by 1KC. Another tranny, similar price but with both windings on one bobbin, was smooth to past 7KHz, so a trice of NFB would have flattened to 20K.

As Bas says: choke-coupled solves some problems, mostly by moving them into another (more money) part.