Vandersteen 5a's - an upgrade from Vienna mahlers?

I would listen to Sean. The Vandersteen 5a is one hell of a speaker and the Mahler's will never match it in the bottom or anywhere else for that matter. The 5a is one of the finest, most technically thought out speakers in existence. Vandersteen does his homework. No way ported bass will compete with the woofers in this system even under ideal conditions. Ports will not do bass as well as a sealed system (especially one as capable as the Vandersteen.)
I have heard both speakers and, sorry, it's just not there. I hope to have a pair of these in my sound room by summer.
But, what is great about audio, you can decide for yourself.
There are plenty of choices.

With the Vandersteen, the woofer amp and all components can be field repaired or taken out and returned to the factory if need be. You don't have to ship the speaker-thank God! Vandersteen has been in business since 1977. I really don't think he will be going anywhere until his retirement considering his success to this point.
As I said, each to his own. However, I personally will buy the Vandersteen's. The 5a's ARE more furniture if that rocks your boat. However, I don't think the rest of the Vandersteen line is that bad looking compared to some of the other offerings in the market (ie; electrostatics)and I'm not in this for show. I for one buy for sound not furniture. If the two coincide, so be it.
As for ported vs sealed bass, the only place ported will win out is in shear volume---that's it.

I know of NO manufacture that will not say his design is the best compromise. However, as Sean says, some basic research is required into such design when you are going to spend this kind of money.
I can only say from my research that [I] will stick with sealed designs. I have spent considerable time with the 5a. It does not have a small sweet spot(of course, personally, I haven't found any Vandersteen to have that small of sweet spot if set up correctly.) It is dynamic, transparent and has some of the best bass I have ever heard from a speaker system. It also reproduces voices as good as I have ever heard with the proper height and width and a amazing holographic soundstage. It will play considerably louder than other Vandersteen's(if you are in to that.)
By the way, I have a set of Advent "The New Advent" (circa 70's) and let us hope we have come a little ways since then. They were excellent in their time but not up to todays standard. But even back then, a ported speaker couldn't deliver the bass it would.

Unfortunately, you can't put up all the technical stuff with a Vandersteen because setup takes some doings. Let me ask this one simple question, when going to a dealer, how many dealer "Associates" have ever measured your ear height at the listening position? If the speaker is not tilted back and set up to YOUR ear height, you will NEVER hear what any Vandy speaker truly offers!

Mikej, Have to totally disagree. The tilt back is one of the most important aspects. If it doessn't make a profound difference in your system, you are probably suffering from some room reflection that are masking some of the differences. The window is really not as big as you think. Talk to Richard V. if you don't think tilt is important.
The other aspects get bass correct and the soundstage focused. Tilt gets the integration correct.
I can change mine with a 1/4" difference. It begins to lose high frequency focus. There's one spot where you don't have a question it's right.