How change the sound of your D200mk2 after inserting in chain the Tortuga?
I wouldn't look to a preamp to necessarily change the sound of an amp. Instead, I'd look for a preamp that passes the signal from the source to the amp with as little degradation as possible. The reason that I switched from an Audio Research REF-3 to a Tortuga LDR1B is that the Tortuga had better bass, both deeper and richer with better instrumental timbres. The mids and treble were cleaner and again with more musicality and the ability to differentiate instrumental timbres. The treble is very extended, but especially it is cleaner than the tube preamp even in the upper treble which surprised me (the treble is after all the home field of tube amps).
The Tortuga really excelled in left to right imaging. By comparison the tube amp had audibly more channel crosstalk and sounded nearly monophonic in comparison. Front to back depth in the Tortuga was every bit as good as the tube amp. That, combined with the excellent left to right image width resulted in a huge soundstage.
One feature of the Tortuga is the ability for the user to adjust the input impedance to best mate with a source. It can be adjusted to any value between 1 and 100k ohms (2 to 200k in the balanced models). You can save up to 5 settings and switch between them from the listening position using the remote. After trying several ranges, I've settled on settings of 14, 16, 18, 20, and 24k. Below that range the source starts to have to work too hard. Above 20k with the DAC I'm using and the sound is very controlled, but starts to sound toward the lean side. Front to back depth also starts to shrink. Right now, I've settled on 18k. In a system with which you're very familiar, you can hear the difference at each impedance increment.
By the way, using another DAC as a source, the sound at 20k was a bit rough sounding with an optimum sound at around 40 to 50k.
All this with an absolutely quiet noise floor and a price (for the LDR1B balanced unit) of around $1800. Worth a try.