MULE - Rick chose a funny acronym. lol. It stands for macdizzy, ultra low end I believe. I don't have the paper, so I'm not sure what his specs are, but a bottom end build usually consists of port timing that suits a longer tuned length pipe (and vice versa), maybe some higher compression and even possibly a smaller bore (than 38mm) carb depending on how important wringing the low rpms are to you. A powervalve is here to broaden the powerband rather than it being developed for a more narrow rpm range. By itself it doesn't give you more bottom end - just allows you to spread it out a little, which is a nice option if versatility is important to you. The tradeoff is cost and a little extra maintenance of the valve...

A good builder should be able to ask you the right questions to get you into an engine that gives you what you are looking for. You have to consider, build budget, maintenance schedule, diversity of riding application and the riding style and preference of the rider. It's challenging potentially to get that information exactly right, but getting a nicely performing engine out of the TRX within your acceptable guidelines is doable.