One other option I considered would be to buy a speaker kit and build a pair of towers. If I go that route I'm gonna want something that'd make me rethink what a good speaker should sound like. A clear step up from my Ultras. I don't know what such a kit might cost tho, or even where to start as far as a style, best drivers for my use...
There's not much that'll scratch an itch like building a pair of speakers. I built a pair of towers a couple months ago and it has definitely been the highlight of my home audio experience. Not just for the fantastic sound, but getting exactly what I wanted for an incredibly low price, the "I built that" feeling you get of course, and did I mention fantastic sound?
When you say "I'm gonna want something that'd make me rethink what a good speaker should sound like." you're really on the right track. While you should have no problem getting a clear step up from your Ultras in "sound quality" for a lower price, you're going to want to put a lot of effort into choosing the right kit because it's not just the cost you'll be putting into it--but all the time and effort as well. You want to choose wisely.
Once you get to a certain "point of diminishing returns" in "quality," IMHO personal preferences take over. This is where you need to figure out what kind of sound you're looking for. Once you have a reasonably flat on-axis response and well controlled, smooth off-axis performance, one of the largest differences between speakers of equal "quality" is just how much those off-axis curves fall--the width of the dispersion.
Have you ever listened to some really narrow dispersion speakers? Some people really love them for music, many don't. Really wide dispersion--various ribbon designs, etc. Your Ultras have very wide dispersion across some of the frequency range but it's not well controlled (at least on the bookshelves, I can't seem to find measurements on the towers) as it is on a speaker like the BMR.
Well controlled happy medium more constant-directivity? Speakers like the JBL 5 series and new HDI series are well controlled medium-wide dispersion. If you like those, you'd probably like the DIY speakers with SEOS or similar waveguides.
Many of these choices depend upon not just your personal preference, but your personal preference in a specific application. When do you care the most about sound quality? Listening to 2-channel music? Movies? Do you like multi-channel music or do you upmix? It's possible your own preference would be different for each application. Are you always alone or are multiple listeners a priority?
And then there are the questions of output capability. How important are dynamics to you? Do you often like to listen (or watch movies) loud? How far away do you sit from the speakers? You have a quite large room--well into the size where Floyd Toole suggests if you like high volumes, you're really better off with a compression driver and a horn than trying to make a speaker with dome tweeters provide believable dynamics and still sound good. Would a speaker that can provide a "full body experience" (as he calls it) put a bigger smile on your face than anything else?
Those are some of the questions I'd give a lot of thought to before selecting a kit. You might not know the answers to them yet and that's fine. Listening to different types of speakers will help inform your thoughts on these aspects. Unfortunately this virus makes it difficult right now, but most DIY guys are very happy to show off their systems so you may be able to find somebody in your area who has built a kit you're interested in where you could have a listen.
Once you have something of an idea of what type of speaker you're looking for, it'll be much easier to zero in on such kits and research them to death.
