I use the Soundtraxx DSD-100LC decoders in my Athearn BB locos. I have had good luck with these decoders and the sound pretty good.
Most BB locos have enough room in the shell for the decoder and a 5/8 to 1" diameter speaker, depending on the model of the loco.
I have also used a few of the low cost MRC diesel decoders in BB locos. They are about the same size as the Soundtraxx decoders and sound pretty good too. I have never had trouble with them and a couple are approaching 4 years of age.
The choice is yours. I personally don't think a BB loco is worth a Tsunami or QSI decoder, but they should work as well.
Good luck with whatever you choose. Once you have used sound in a loco, steam or diesel, you will never go back.
Remember its your railroad
Allan
Track to the BRVRR Website: http://www.brvrr.com/
Either QSI or Soundtraxx would be the best IMO. Try their site. They have decoders specific for the loco types you noted.
Springfield PA
What does everyone recommend for decoders with sound for the Athearn "Blue Box" units, GP38-2, GP40-2, SD40 and does anyone have a link to an example of an installation of them as well.
Thanks...