You don't need a separate amp, but the unit is a separate component. I am not familiar with the 3090, but if it is a preamp only, then you WILL need a separate amp whether you have the DTS decoder or not. The 3090 will also have to have preamp inputs in order to connect to the decoder, which means you will need 3 more interconnects.
All in all, it would likely be a better path to simply get a newer receiver that has DTS.
Interesting, that must mean they have a module that can add DTS to it, or it is firmware/hardware upgradeable. Does that mean it does not have 5 channel output (L/C/R/LS/RS) right now? AC3/DD does not necessarily mean 5 channels, but I don't think I've ever heard of DTS with less than 5 channels.
I looked at doing the mod a while back. It is a hardware mod, the 3090 has pre outs for L/C/R/LS/RS and front effect outs x 2. It has inputs for L/C/R - no surround channel inputs. The mod involves changing the front effect pre out jacks to rear L/R in jacks. I cant remember the cost of the job. If you do the mod and connect a DTS decoder, to use the 3090's internal DD decoder, or if you want to use another source such as CD or video etc you need to remove the plugs from the back and re-insert the jumper bars between the ins and outs for the L/C/R channels. I was planning to use my sony DVP-C670D dvd player with DD/dts decoder, but the crossover was fixed at 120hz, in the end I figured I'd get better results out of the DD decoder in the 3090 than I would with the DTS out of the sony, so I didn't go ahead with it. It *may* be possible to add DTS by changing some internal parts to DSP-A1 parts, but I've never heard of anyone doing it that way, and I dont think it would be cheap. In the end I bought a AVR-3803.
Uh, actually, you can get the 3090 converted without having to do all of the extra amp business and such. Go to http://www.msbtech.com/Catalog/FAQ/adddts.htm It's $595. These conversions have been available for several years...
Personally, I would not do it even though MSB has an excellent reputation. I had a 3090 and while it was a nice piece, there are bigger fish in the sea (newer, better features and user friendliness.) I would put that $$$ toward a new unit.