I have an Oto SE, and in my opinion it is a pretty warm sounding amp out of the box. Mullard EL84s will sound warmer than the stock tubes, but IMHO there is also a loss of the pleasant clarity and timbre or grain to the music.
I have tried new tubes in almost every position, and the amount of change they bring to the sound varies from a lot (EL84s) to almost none (6922s in the phono stage). I have had good luck with CBS Hytrons and (surprisingly) with Baldwin 6BQ5s; which are still quite a bargain. The latter are hard to find matched, but they are cheap so buy many and match 'em up yourself.
I have tried new tubes in almost every position, and the amount of change they bring to the sound varies from a lot (EL84s) to almost none (6922s in the phono stage). I have had good luck with CBS Hytrons and (surprisingly) with Baldwin 6BQ5s; which are still quite a bargain. The latter are hard to find matched, but they are cheap so buy many and match 'em up yourself.