Hi jamme61,
New to this forum, and Egnater amps. Not an amp snob or anything, but my favorite is my HRDlx, and the Twkr 40 gives it a good run for it's money.
I know that this thread is a little old, but thought that it deserves an answer and I just happened to buy both recently. You are correct that as well as more power in the 40, the 15 is Cathode biased, and the 40 is fixed. But another difference as far as sound is that the 15 has 6V6 and the 40 has 6L6. The 40 is also two-channel, which is a big plus. The 40 can get very close to the 15 sound, and both have identical controls, the 40 adding more controls for two channels. I found the 15 sounded great, really nailed the sweet 6V6 tone that I like, but no channels to switch sounds, and limited in clean volume before distortion. Plus, as Tim Allen says; "I needed more power, o-o-o."
I have been looking for a small amp head with that 6V6 sound and after trying the Rebel 20 and 30 heads, I bought the Twkr 15. Although a great sounding little amp, for me it was just lacking a little bit as mentioned, so I traded up to the 40. Very pleased with the fact that the 40 can get very close to the same driven sounds as the 15 with the addition of the extra switches on the 40, and also that the 40 has a much better, louder clean as well as the included channel/effects foot switch. I use an 8-ohm HRDlx 1x12 ext cab and it seems a perfect match for the 40, and the amp probably would sound even better with a 16-ohm cab, maybe even a 4x12.
The controls on both 15 and 40 are identical, extra control switches on 40, and seem very familiar even though this is my first Egnater, I just like the layout a lot. In the end I didn't care for the blend or variable watts features on the Rebels, and I found that reverb isn't essential, so the 40 was just what I was looking for in a small, light, powerful amp head. If you just want great 6V6, Cathode biased, creamy distortion, go with the 15, and for more versatility and power the 40 is great. The weight difference between the two isn't much either, and about $200 price difference.
I took the 40 for it's first trip out of the house for an impromptu jam at friend's last night, and it is really a very nice sounding amp, and so far has great reviews. Time will tell as far as reliability, but it seems well constructed and I have confidence in it being solid.