On the Spiros question - I'd gone with Spiro weich based on the input of a few local bassists of prominence, who for the most part got it from another local luthier, who advocates strongly for those strings. I'd heard them over the years on a few basses and never liked them, always thought they sounded thin and harsh, but I let myself be swayed. A $260 mistake. With a decent sounding bass, BelCantos sing nicely.
As for the soundpost adjustment thing that's largely up to your luthier. If you've got a guy or gal who's capable of intuiting the proper location for your regular bass soundpost to your liking, chances are they'll have a decent grasp of what's needed with the relatively less efficient Chadwick. Don't get me wrong, I really appreciate what Chadwick has brought to the community! But the big rectangular hole in the back and all the extra wood and plastic inside don't do ease of vibration any favours. So in my opinion, which isn't often terribly humble (sorry), moving the post further from the bar to give the bridge more mechanical advantage in driving the belly on the lower notes seems obvious. And that's done by moving the fulcrum (the post) further from the bar (the end of the lever arm) to bring a broader amplitude into play.
Generally speaking, arco-centric bassists usually want their soundposts set approximately symmetrically with the bassbar of their instrument. Maybe a little further from centre if they play the odd jazz gig. Pizzicato-centric players usually favour having the post set somewhere between 5mm and 15mm further from the centre seam than the bassbar. It's not great playing a bass set up like the latter in a classical setting, which is why dialogue with the player about their repertoire is so important. A few millimetres can be a big difference in terms of sound quality and control. Go too far, and everything just goes to mud with a bow. Too conservative (too close to centre) and the thing just goes *thop* when played pizz. It's between you and your luthier, really, but that's the basic groove of the thing.
As for the pins; Chadwick just puts tiny finishing nails in each end of the soundpost and they stab into the wood of back and belly, to preserve post location while the thing's being bounced around on a plane. I prefer a foam ring lightly glued to back and belly at each end of the soundpost, something less damaging and easily removed if necessary. But the pins work. And Chadwick's protruding pins are less invasive/destructive than La Marre's finishing nails driven right through the back and belly, as I witnessed on Occipinti's bass some years ago. That was scary. But he swore it was fine, that his old bass was so stable it never needed a post adjustment... okay...
Squeezing the bass belly from side to side with a clamp makes easing out the pinned post not too hard. Then pliers pull the pins. The luthier proceeds to adjust the fit to a more advantageous position, marks that at each end (very easy thanks to the opening in the back), and the pins get re-inserted, the belly squeezed from side to side, the post re-inserted and the clamp let go. Easy-peasy.