Be careful what you ask for with the Merlins. Along with fat, full and warm you most likely will also get slow and syrupy. I have 2 pairs of Merlins and have run them with several different amps, including some older CJ amps. With Premier 12's and a Premier 16 pre, for example, the sound I believe was as you describe you want. But along with warm and full I did get a much slower sound. Warmer tubes, like Mullards, accentuated it even more.
Currently I'm using an Atma S30 with one pair and a Pass Int 60 with the other and like both more than the older CJ sound but that's me. I don't believe you'll get to where you want to be with an OTL, but with Pass you might. See if you can find an older Pass Int 30 unless you have a large room or play loudly. Then the 30 will be under powered.
Good luck.
Currently I'm using an Atma S30 with one pair and a Pass Int 60 with the other and like both more than the older CJ sound but that's me. I don't believe you'll get to where you want to be with an OTL, but with Pass you might. See if you can find an older Pass Int 30 unless you have a large room or play loudly. Then the 30 will be under powered.
Good luck.