After reading about the success that F1nut & DarqueKnight had with the 4.1TL modification to their CRS+'s, I decided to try it on the SDA 2B's. The modification is simple and involves adding a 5.8uF capacitor in parallel with the 2.7 ohm resistor and replacing the tweeters with RD0198-1's.
Previous Modification: I previously upgraded the crossovers with Solens for the lowpass (20 uF and 39 uF), Sonicaps for the high pass (12 uF Gen I with 0.1 uF Gen II bypass), and Mills MRA-5 2.7 ohm resistors. The tweeters were replaced with RD0194-1's.
TL Modification, No Bypass: I installed Sonicap 5.8uF caps, removed the 0.1uF bypass caps, and installed RD0198-1's.
Listening Impressions: I immediately noticed that vocals were more clear. One good example is the Alison Krauss + Union Station Live CD. There is one passage on that CD where the vocals had a slightly rough edge. As soon as I heard it, I realized that there were two voices and both are now clear and distinct. The RD0194-1's sounded really nice but a little soft for my taste. These are both crisper and smoother at the same time. Bass is better defined, mids are more clear, and the highs have more detail. Overall a big improvement but they seemed a little dry and clinical compared to the old setup. I left them playing all day while at work and listened again. Still a little dry and clinical.
TL Modification, Bypass Reinstalled: I reinstalled the Sonicap 0.1 uF Gen II bypass caps in parallel with the 12uF Sonicap Gen I caps.
Listening Impressions: The bypass caps add a little bit of a sweet and airy sound that really works well in my system (all solid state). Now the sound is similar to before with all the improvements noted above. Several members have decided to remove the bypass caps in their SDA's when using a Sonicap high pass cap. I can imagine the bypass caps might be too much in a system with tubes. I'm not sure what experience others have had with solid state systems and bypass caps. In my system and to my ears, though, the bypass caps are definitely a keeper.
Amplifier Change: On a side note, I upgraded my old Yamaha integrated amp to a NAD C370 before I made this modification. The Yamaha sounded fairly respectable with the RD0194-1's although a little rough around the edges at times. The RD0194-1's sounded smoother but a bit too soft with the NAD C370. Now I have the sparkle and the smoothness with some added punch.
Thanks: Thanks to F1nut & DarqueKnight for the great posts on the 4.1 TL modification to the CRS+. Thanks to F1nut for the schematic. Thanks to Ken Swauger at Polk CS for reassuring me that this would work as well as on the CRS+.