Building the Audio Note Kit 1 SET amp...

Tube you are right of course. I recall going through the risks with reb beforehand. We were all hoping for the best but knew the risks. So no toap surprise i suspect. One just has to complete the journey. Often one big move calls for followups before all is well. Powered subs blended well will add cost but resolve the problem and retain and likely even improve further the good things currently heard. Changing speakers runs the risk of additional trial and error until the right ones are found. Chances are they will not sound just like decapos but maybe even better once overall good karma happens

These kinds of problems is what makes this stuff fun as long as one has patience and proper financing. :^]

With subs its up to the buyer to do the integration to their satisfaction. It takes some time and discipline but is doable usually. Ive done it to my satisfaction in the past but it required a lot of focus and a clear goal it can be done by ear or with help of tools designed for the task. Rebbi built his own amp so i suspect he would get it done well. Its just audio though not life and death. Lots of ways to skin the cat.

Regarding the possibility of adding a sub, in addition to the concerns about integration that have been raised I would have some concern that the high pass filter and other circuitry in the sub, that would be in the path between the preamp and the SET amp, might noticeably compromise the SET magic.

And then of course there is the cost of the sub, which would be purely an additional cost rather than a cost that could be offset, at least partially, by selling the existing speakers or amp.

FWIW, although under the great majority of circumstances I am in the "build the system around the speakers" camp, in this particular set of circumstances, all things considered, as I indicated in Charles' system description thread where this has been under discussion my vote would be for a speaker replacement.

Best regards,
-- Al