Bass clarinet random thoughts
In the way of qualifications, I offer the following:
I've been playing bass clarinet since I started instrumental music in the late 1950's. I own a number of basses, and play them as a professional musician. I started with AFM and professional music back in the 1960's. Plus, I have a full box (one of those copying paper ones) of bass clarinet "stuff" like mouthpieces, extra pegs, ligatures, swabs and the like. Five decades of bass clarinet time should equate to a little knowledge on the topic.
Having said that:
Used bass clarinets of whatever price point almost always have spent some institutional time. (Few individuals buy them for themselves, and those horns are almost always guarded treasures.) Schools and colleges are notorious for beating the crap out of harmony instruments. Keep that in mind when you are looking, and don't let a good price convince you to buy an instrument that may be cracked or otherwise damaged.
Used and good and still affordable usually means an older Selmer horn, to low Eb rather than C, and the price of an overhaul on top of the purchase price. This approach would give you a horn comparable with your rather unique soprano clarinet.
(I would not buy a Kohler, as I have found the key work on them to have been "worn out" with no room for adjustment. In any event, most of them have passed from the realm (they were popular horns in the 1950's), so the chances of you running into one are way down these days.)
(Although I own a couple of them, I do not recommend extended range bass clarinets to casual users. You are paying a lot of extra money for very limited utility. If you can get a good one cheap, this advice goes out the window. But, you won't find one (except the Chinese abomination) at a cheap price point unless it has been flayed to within an inch of its life.)
(I would also avoid any bass clarinet that has a peg support physically attached to the wood of the lower joint. (This usually means two hollow posts with thumbscrews that bear on the peg rod.) The combination of poor installation with careless handling and bashing the peg rod into chairs and such makes this a bad thing.)
By no stretch of the imagination is a used Leblanc pro horn the equivalent. Up until the last few years, all Leblanc horns, whatever the price range, used the "register key on body" setup without a linkage to the lower joint. (The easiest way to determine if you are looking at one of these older Leblancs is to look for a second bridge key with a long rod running up the right side of the upper joint when viewed from the rear.)
A Leblanc instrument will play well enough except for when making jumps from the lower register to B, C, C# and D in the staff - this interval does not articulate as smoothly as notes on the rest of the instrument. Some will disagree, but they usually are those who only have access to a Leblanc instrument and are making the best of it.
Other than for that (and the horrid fork Eb mechanism that some of their horns had on the lower joint, and some minor ergonomic issues with the key work), Leblanc horns are just fine. If you are looking at playing "oomph-pa" parts (concert band stuff) in the lower register, one will work just fine as long as the fork Eb mechanism is maintained. But, for anything approaching regular clarinet facility, Leblancs are a no go.
(I leave out Buffet horns here. If you can find a used one for a reasonable price, I'd buy it. But, they just aren't there to be had. Also omitted are Yamaha bass clarinets. A good enough instrument, but I haven't seen any on the secondary market.)
(Others, of course, may disagree...)
If you want to step down from "good" to "good enough":
Once you drop below the level of the "professional" (meaning wooden, Paris built) instruments, your choices are much less varied. There is the "Chinese" bass clarinet being imported under a variety of brand names (Kessler is one). I have fiddled around with these and would not buy one on a bet. Poorly made, poorly aligned, and very likely a repairman's nightmare.
Other than things Chinois, there are the various "student" level basses. On these, you automatically will get the register key on body setup. The old Bundy horns (Selmer's student brand) were notoriously hard to keep in tune, not something that I have found with the Vito brand (Leblanc's student brand). Yamaha also makes student basses, but I have never been able to play one of these.
In any event, you are going to get a horn that is a lot more rugged (particularly for the big keys on the lower joint) and impervious to weather. It will also be a lot less expensive.
Executive summary:
In the student end of things, I would opt for either a Yamaha (based upon reputation) or a Vito. In the pro end, I would purchase a used Selmer (after first making sure that it was crack free).
If you could buy new, my recommendations would run Buffet (pro) instead of the current Selmer. Student would be Vito or Yamaha. (There haven't been any intermediate horns (Selmer's Signet, and Leblanc's Normandy) in a long, long time.)
One more piece of advice:
When you buy a bass, also invest in a proper instrument stand. You'll need one in any event if you play in musical pits, and keeping it in a stand instead of setting it down on its side will ensure that those keys on the lower joint stay in alignment. About the only one that's available today is the K & G "bassoon/harmony clarinet stand". It's a bit cumbersome, but that's all there is.
Incidentally, there's a whole book out on bass clarinets (and other harmony instruments, including the musical abomination that is the alto clarinet. It's titled From the Clarinet D'Amour to the Contra Bass: A History of Large Size Clarinets, 1740-1860, and thoroughly covers how Sax saved the bass clarinet from being an ungainly monster.