1) You should definitely consider the purchase because the only way to know if tubes are for you is to try them in your system.
2) Upgraded tube-quality sound? Only your ears can tell you that.
3) A tube preamp is a great option if you're otherwise happy with your Emotivas.
4) A tube preamp should have an output impedance at least 10X lower than the input impedance of your Emotivas. Otherwise, you just need enough inputs (including built in phono if you want it) to hook up all your sources. I'm a fan of the 6SN7, but there are great preamps that use other tubes too.
5) Triode/ultralinear switching is nice, but my experience is as long as triode mode has enough power for my speakers I prefer it with all types of music, so I rarely switch. I've had the Willsenton R8 in my second system for about a year now and after the initial experimentation I've left it in triode 100% of the time with my Caintuck Lii 15s.
The Rogue and Cary units are good choices and fit well given the rest of your system; $3.5K for preamp alone might be overkill.
New DAC is a whole 'nother discussion. I'm 95% vinyl, so my Schiit Bifrost is more than enough.