I would say that a well designed SET 2A3/300B has the warm sound than also well designed an OTL having the same output wattage.
There really aren't any OTLs that make the 2-7 watts as suggested by the above post. OTLs become less practical at lower power levels- driving the real world impedances that one might do with a transformer-coupled amp gets harder. That is one reason OTLs tend to have a lot more power than SETs, and is why our smallest amp makes 30 watts into an 8 ohm load. To make the amp much smaller than that becomes impractical.
However I have found that the OTLs tend to make less of what I have come to call 'loudness cues', IOW higher ordered harmonic distortions (which show up in most SETs when driven over about 20-25% of full power). The result is that the OTL won't sound as loud (even though it is probably louder) and so invites you to use the power it has. This can be done comfortably. The reason SETs sound so dynamic is due to distortion and how that interacts with the ear, not actual volume!