Scott,
A well-designed 845 amp that gives you dynamic muscle with Def III without losing any of the involving intimacy of SET is, for the most part, beyond your budget. If you get lucky in the used market, terrific. Any of the big glass amps that are designed well for the challenges of the tube tend to hold values above $3500, and integrateds are scarce. In the related 211 realm, the Melody 211 integrated drives Defs well, but it is a $6000 amp new, with used showings so far exceedingly scarce.
Definition - any version - poses specific challenges. Prime is that unlike pairing a tube amp to a Zu speaker limited to the low-end response of the Zu FRD, with Definition you have the sub section pushing out bass energy down to 20Hz or lower, with the internal amp deriving its input signal from the main amp output. So the sub takes on the bass characteristics of the main amp. A big-glass amp with insufficient power supply, or a lower power triode suffering bass bloat, will generally prove unusable with Def, while sounding perfectly fine with, say, Omen or Superfly.
At your price range, barring getting especially lucky in the used market for a suitable 845 or 211 find, consider abandoning the big glass at that price as well as an integrated. You can more readily find the next best thing: a pair of Quad II mono blocks (each of which is smaller than a shoebox) and add a decent preamp. With used Quad II pairs or used Quad II Classics from the current production line available on the used market in the $1200 - $2200 range, you have cash left over for a good preamp -- or a TVC (from diyhifisupply.com). The Quad II is the most SET-like push-pull tube amp I know of, and one of the simplest tube amp circuits available. Just tube it with the excellent Shuguang Treasure KT88 and find a NOS pair of EF86 input tubes, along with a good NOS rectifier. It has good bass control for Defs, especially since with Def3 you get the Def4 bass EQ.
You also might look into the Mystere tube integrateds, built for that brand by the Melody folks, if you must have an integrated.
If you have a front end with volume control, you can be patient an wait for a pair of the first-gen Sophia 845 mono blocks to show up on the used market. They're usually around $4000 used for the 206-driver version (less for the 6sn7 driver version) and bring the full game, including proper power supply, parts quality and transformers.
There are some alternatives that are well under your budget, as well. $3500 just happens to be something of a no-man's land for getting big glass + integrated + def-worthy design and build.
Phil
