With respect to your experience with these instruments, could you be more specific?
So here is why the Kala California is a gigantic improvement over the others I've played:
1. Sound - Kala uses LR Baggs active electronics. The pickup is higher quality than anything else on any of these small basses. If you use a quality amp you can really hear the difference.
2. Balance - The Kala California is wonderfully balanced. No neck dive.
3. Small body - Very comfortable to hold. You don't need a big body with a solid body instrument.
4. Neck - The neck feels great to hold. Fret ends are properly finished.
5. Frets - Kala uses very high quality frets on the instrument.
6. Overall Fit and Finish - Mine is fantastic.
When I play the Kala California, everything about it feels right and it sounds great. It's in a whole different league from the Asian made Kalas - which are not bad at all.
I think the Rondo acoustic is a very good deal for the price although it is quite unrefined. The Rondo solid body that I had had simply horrid electronics. The Kala Sub-U is a great little bass, with a very different sound from the California, but the balance and neck are nowhere near that of the California.
I am as guilty as anyone of buying a lot of junk when the same amount of money would get me something that's really good. Of the ones I have owned the Rondo solid body and the Gold Tone solid body particularly deserved to be called junk. Although in fairness to the Gold Tone, even Gold Tone agrees that the one I got is defective. But from a design perspective I'm not fond of the neck or the weight or balance. The Kala Sub-U that I bought is a bass that I love despite it's flaws, but I have spent some money to upgrade it. As it originally came, the pickup jack was just too cheap.
I have not experienced the ShortBassOne. At this point if I were to recommend for a first time buyer I'd say this:
1. For a really cheap U-Bass to see how you like it and don't want to spend much - the Rondo Acoustic is the "best" buy. It has a lot of the faults of the Kala Rumbler (cheaply made, don't like the strings, no access plate in the back). If you want the cheapest it's a better deal than the Kala Rumbler.
2. Kala Acoustic solid top (not the rumbler). This is a few hundred more and worth it. Much higher quality. Better strings (the Pahoeohe strings work overall better than the silver strings). Back access plate. My preference is for the earlier ones with passive electronics.
3. Kala Sub-U Bass - I love mine. In fact I prefer the solid body U-Basses for the size. You need to plug in anyway. Also no feedback issues. Access to higher frets. The only reason I don't recommend this for first time buyers over the Acoustic is that it is a solid body. With some ukulele groups you have to have an "acoustic".
On the other hand if you are have some experience with the bass and you know you are going to be playing it, and you don't mind spending a bit more, the Kala California is more expensive but in my opinion well worth the extra dollars. Analogizing to ukuleles - many people feel that a Kamaka is well worth the extra dollars over a Kala. Most of the U-Basses are comparable in quality to a $100 ukulele. The Kala California is like moving up to a K brand.
Also while all the Uke Basses look very similar, there more differences between them than between one ukulele and another.