Your amp I'm guessing? Amps don't make a huge difference in my experience... The NrT upgrade is exponentially more significant than having a non-overdriven decent amp and replacing it with an expensive one. Speakers basically make all of the difference.
But yeah now since a new upgrade was announced, might be a good idea to hold off on an NrT upgrade... (maybe Dave can weigh in on the matter to see if this is indeed the case? I'm guessing probably...)
I guess developing or modifying an existing woofer would be viable for for a minimum number of orders... As the NrT tweeter was used for the Sierra NrT upgrade and the towers, it makes more sense than a maybe very limited of custom midrange driver for a Sierra-1 upgrade... Costs add up especially with the cabinet, so maybe finding an existing mid (and maybe modifying slightly?) might be the solution...True, one could use a different midrange transducer. Perhaps SEAS or another OEM just finished baking a brand new midrange woofer with the exact sensitivity Dave needs for a 3-way Sierra-1.
hehe SE for woofer might be tempting too... Not sure how doable/practical it would be, or the performance gain... But I think most modifications would require crossover modification, which would mean a crossover for the regular Sierra and one for the NrT... The mid cabinet too I would guess for the original Sierra, but at that point, a NrT upgrade might be necessary, so maybe would be the same for this 'SE' woofer upgrade...To be honest, I'm kinda out of ideas, so I probably won't say too much more...that is until Dave gives us another bone to chew on.
Fact is, there are only a few possibilities since this is a DIY upgrade. It has to be reasonably simple (for customers), but offer "significant" improvements. If you're right GirgleMirt, I'm sure there will be a lot of happy customers. I just haven't decided to toss the proverbial coin yet...
Heads: new tweeter/crossover
Tails: "SE" type upgrade
Edge of coin: mid-range upgrade