Just a reminder, we don't have a double bass, but are looking to get a double bass-type thump out of an electric or acoustic-electric bass guitar. :-) We agree that fretless would be better, but we don't have it. Yet.

Thanks for the continuing discussion. Dan and I will be studying this and get back with further questions for clarification.

And, just to make the mix more interesting (on the "affordable and portable alternatives to double bass" front), something some friends have told us is cool is a ukelele bass (Kala Brand U-Bass). These are strung with a thick polyurethane string, and with a 21" scale (a baritone uke body) they give the same pitch as other basses. Apparently they sound surprisingly good.
manufacturer video, mentioning various pros who are using them
happy jazz user video

~ Becky in Tucson