The night before last, I finished installing the TSU-1000 Tsunami decoder into one of the two Atlas Gold SD35, QSI Quantum sound locos I have, retaining the twin (1 either side, facing each other) 30mm round speakers they have in a cast metal enclosure that straddles the rear driveshaft and firing down past that shaft.
I had previously installed another of the TSU-1000 into a Silver DCC/sound ready SD35's, (same twin speaker mounting supplied), with the iP6s plus speaker, sandwiched in between the decoder and the Dynamic brake fans. I had removed the thin plastic baffle from under those fans. As the iP6s speaker exits the sound from the end, that was forward of those openings and close up towards the Cab.
Tried each individually on my 6 foot test track. There doesn't seem to be a great deal of difference (neither have had any tuning as yet) except for the horn sounds. The iP6s was not loud, where as the bell was. The one with the original twin 30mm speakers horn was loud, to the point of a bit of distortion, bell, about the same. My club is closed, so I won't get to try them out fully till it re-opens, unfortunately.
I have one more to do, with the same decoder, a DCC non sound ready version with a solid rear weight, so that too will get an iP6s in it. I may cut some material from the top of the weight and place the speaker there. I do this with Athearn's SD40's with the step down in the speaker overhanging the decoder on top of the motor. A bit of insulating tape on top of the weight to insulate any bare metal on the speaker and give a slightly padded seating and a strip over it and down onto each side to secure. I use ordinary Scotch tape for this to keep it thin, and not interfere with the shell sliding down over.
I'll put up some pics later to illustrate.