Depending on where you live, there may be other media extenders supporting DTS downmix availble at a similar price point. So you may not need to confine your list to only include the Xtreamer.
As far as standalone decoders, they are a bit hard to come by nowadays. Three that come to mind are the Technics SH-AC500D (which may be listed as Panasonic in some regions), the Klipsch ProMedia DD-5.1, and the CreativeLabs DDTS-100. The former two support DD, DTS, and DPL. The latter supports DD, DD-EX, DTS, DTS-ES, DPL IIx, and DTS NEO:6 iirc. Also, I believe the former two use standard RCA outs, while the latter uses miniplugs (there are readily available miniplug to RCA cables).
These have 6 or more RCA outputs, as they are intended for use with the multichannel analog inputs on older receivers. However, I suspect they can be configured for downmixing, as that is a pretty standard feature for DD/DTS decoding. I would recommend you look up info on that though.
Since they are a bit older, finding them new is difficult. And when you do, they are typically overpriced. However, you can find them used if you look around. For example, there are a couple of the Technics on ebay as we speak.http://shop.ebay.com/items/technics%..._dmd=1&_sop=12