I don’t know what would be best for your Bachmanns but the following thread may be of interest where folks commented in general on their opinions on various decoder types.
http://cs.trains.com/mrr/f/744/t/214272.aspx
I found the advice helpful when I wanted to convert a Kato RS2 DCC ready (8-pin) to non-sound DCC. I chose for that case a TCS, I think only about $32(?). I was not ambitious enough to attempt the tight fit speaker placement in an RS2.
As I’ve got into consisting, I’m not enamored of the Soundtraxx Tsunami diesel decoders (in my Genesis GP9). The horn is weak (may be part the speaker weakness) and the lack of CV6 (max speed) and CV5 (mid speed) adjustment mean speed adjustments have to be more complex (using CV29, CV25, etc).