I plan to replace the 075 with a new modern compression driver (Radian 475) matched with a DDS waveguide. The combination has a usable frequency response of 800-20K, but requires a 4th order xover at 800 hz. I do want to cross as low as possible since I believe that will improve MR clarity.

Based on the factory spec of the D131 (I do not have the drivers available for test right now) and the cabinet dimensions (2.52 cubic feet) the D131 will develop a very peaked response at resonance with a standard LR 4th order LP (according to Unibox). It seems in part that this is due to the size of the enclosure.

I'd appreicate any suggestions for successfully integrating the two drivers. Also, I suppose, this begs the question why not use a Pi alignment. I admit that I'm not sure the tradeoff of the higher xover frequency would be beneficial in this case. Again, advice/comments appreciated.

Maybe others will chime in, because I have no experience with your chosen Radian tweeter. I can only say that I would generally prefer to push a subsystem up in frequency than down, if a compromise in this regard has to be made. This is especially true of modern compression horns.

I understood the advantages of choosing those slopes to be less interference between the drivers across a given 3d space. That seems attractive; however, I'd have to move the xover up higher.

Can you give advice on how much of a difference this makes in sound quality and whether you think it would be worth the trade off in potential MR behaviour? See addtl. data below.

I have listened to the Radian driver and DDS waveguide and it sounds good w/ the LR 4th order at 800hz. I will not go lower though. With the D131, the JBL folks claim it goes up to 5K; however, I've not measured it yet (I'm helping someone out of my area). Given its a 12" w/ large aluminum dustcap, I'm expecting some nasty break up modes on the way to 5K. Certainly when I've listened to the system as is, the MR clarity is rather poor.