Some stores are better at setting up saxophones than others. Saxophones are quite fickle to begin with, and it can take considerable time to fix certain problems, so if saxophones weren't so popular (and expensive), I'm sure that most stores wouldn't bother with them at all. Another critical factor that can cause warbling is the fit of the mouthpiece to the neck. If the tip of the neck has any gap around it (inside the bore of the mouthpiece), this can cause warbling - particularly at the low end. For an experiment, you might wna to try wrapping some teflon tape around the very tip of your neck cork. This may reduce or eliminate any gaps and might solve the problem if it's not a pad leak. I brought my personal horn (a Selmer III) into a store once to play it along side a similar model (a Selmer Ref 54) and I got the warbling on the 54 until I put my III neck on the store horn. As my mouthpiece was fitted to my neck, the problem was solved. Sure you don't want to play the drums instead? I'm actually selling mine for $495. www.saxmaniax.com/sax/as07.htm

You seem to have a rolling bottom issue ,not customary but quite common none the less,,, especially in Selmer MarkVI and those misguided companies that copied the design ,,this can actually happen on the high C as well it tends to roll when blown hard ,,,,,Try this to find out where the problem is : put the plastic end cap (the one used when you store the horn in its case ) into the bell ( drop it in ) play the horn ,,if the problem goes away it is not a leak but a flaw in the horn look for another sax

I've noticed the rolling bottom on vintage horns - particularly C-Melodies. I throw my mpc cap down in my C if I have to play it quietly - like in church. I've not noticed this on the YAS-23's, though. The bow design and construction on the YAS-23 is very similar (almost identical) to the Couf/Keilwerth bow design - the Yamaha being only a few mm shorter but the same bore, essentially. If only the YAS-23 had it's tone holes more inline, it would be more comfortable playing seated center. But I digress... I've found the YAS-23 design particularly stable, so I'm betting it's a pad leak or mouthpiece fit.

Thanks for your comments, I've been away for a few days so I'll try inserting the mouthpiece or end cap in the bell tonight.

FYI I inserted the end cap in the bell and played both with and without it in there. There was no difference re the gurgle. I'll wait a few weeks, and if it does not go away, I'll take it back to the shop for adjustments.

Have them put a new neck cork on in case yours is too compressed - causing a gap. Between that and having the lower keys adjusted to eliminate leaks, we'll have to talk about embouchre next...