TubeNet

The closest thing to the alex 164 was the hirsbrunner HB6. We had the alex 164 at Hartt and i stood them side by side with my HB6 and was blown away with how close they were. The PT 6 is also close in size. I will say that the 164 BBb is a great tuba but in CC they are horrible as they were just cut down from BBb.

The 601 is also pretty close but the sound was not the same. I have had both the 18 inch bell and 20 inch bell 601 and prefer the 18. The intonation on the 601 is doable but be ready for a very low CC as compared to the rest of the horn.

I haven't had all of those side by side before, but none of them seem too much like the PT6 from what I recall. The Cerveny in my memory is a lot taller and bulkier. The Alex is more stovepipey and not as fat, but is all around scaled larger and very big. The PT6 is like a BAT but smaller.

The few Cerveny kaisers I have played were okay tubas. Huge, but not particularly interesting sound. One in particular was so resonant, it felt like it was going to explode into a pile of parts... Not good. The 601 leadpipe goes into the top of the valve block.

The Alex 164 I have seen is Ben Vokits' (formerly JC Sherman). Wacky fingering patterns to accommodate the awful intonation. Super mouthpiece sensitive. Absolutely huge and awesome sound... Just takes some work to get it. Ben told me before that the 163 could put out more sound, though not as much extreme weight. Horn Guys has an Alex 164 CC right now. Excellent condition, awful intonation and a beast to play.

The PT6 is the most mainstream of these for a reason. Easy to play, a sound that many people like, and oh yeah, easy to play. That said, the last time I heard one in a professional orchestra, I was left wondering where the tuba sound was. Not the weight and presence Inwas used to hearing... But then again, my skewed frame of reference was hearing Alan Baer and the NYPO for 8 years.

Probably a rambling non-answer to your question...