Hi Brian,
I had much the same system as you with Ellis 1801s and Van Alstine equipment and, in my opinion, it sounds fantastic. I have the parts to build another Dennis Murphy speaker design, the CAOW1, and just need some time and crossover parts to get it done. As you may know, the CAOW1 uses similar drivers as the Song Tower and should sound very similar.
Dave Ellis mentioned that the CAOW1s give up very little to the 1801s (which is high praise from Dave since he only sells the 1801 parts) so I would think that the Song Towers and the 1801s would be very close noting that both are designed by Dennis Murphy.
In my opinion, the next big jump would be to go to a well designed 3-way speaker like the Salk HT3s (also designed by Dennis Murphy), which would provide more bass depth and presence.
Cheers,
Tom