Hello Rich, all this talk on the 'Gon about the new audio codecs really has me in a state of deja' vu. The same sort of thing happened when Dolby Digital and DTS began to replace Pro Logic in the late '90's. Back then, as now, if you had spent good money on an otherwise great-sounding receiver/processor just before DVD came out and it lacked the latest processing ablity or the "Dolby Digital-ready" analog 5.1 inputs, you were SOL if you wanted to try the new DD/DTS formats without replacing your processor.
Fortunately, in your case your Theta has a set of 5.1 analog inputs. If you are not using them for multichannel SACD/DVD-A music from your Lexicon universal player, then you could buy a Blu-ray player which decodes the new formats in the player and hook it up via 6 analog interconnects. You could try the Sony BDP-S550 for less than $400 to see if the new lossless codecs rock your boat. For higher quality and more money, you could consider the Sony BDP-S5000ES, Marantz BD8002 or similar at around $2,000. All of these Blu-ray players will let you configure for a 5.1 system and will downmix a 7.1 soundtrack to suit your system. Of course you can still enjoy old-school DD/DTS from your Lexicon digitally connected to your Theta via a digital coax or toslink cable.
You may also wish to contact your Theta dealer regarding possible upgrades for your Casanova as it has an "open architecture" which should allow you to have it upgraded to decode the new lossless codecs and HDMI connectivity if/when Theta decides to offer it.
Good Luck! -jz