By now you will realise I have been around a while, and was of the generation that had to struggle with making computers work in the studio, fiddling with IRQ jumpers, bios settings and garbage like that to achieve what literally anything... including what even your phone can do with digital audio now. I have lived through every iteration of digital audio so far and struggled to make it sound optimum at every step. I have laid back to tape, used valve interfaces in front of A/D's, tape sims, I guess technically we even "analog summed" (i.e returned the outs to a desk) those black face 16 bit ADATS. I can honestly tell you, despite the romance that summing into a cheap ass or even mid-range desk does NOT give you ANY magic, and sounds worse than any DAW with analog modelled plugins does today, unless you have a (good) Neve, SSL or API to hand, or a handful of other classy beasts. You want it to, but it doesn't. This is good news!
Not long ago, I read a book by a well respected mix engineer and one of the things he said was that you NEED an analog summing box if you want to get pro results. Because like...the uh, summing math. Bullsh!t. Unless you want to interface analog gear into your setup a little more effectively - a GOOD use for such a device - you do not need a summing box. You will read such statements such as it bringing "increased depth" "wider soundstaging" and being "more three dimensional" when it comes to those who are proponents of summing - as usually, they have spent a ton of money on it. In the real world, it makes minimal difference if you set your mix up right, and even then only on certain kinds of mix, and not for the magical voodoo reasons. When I moved up in the world and got to work in nice places, I bussed out to the desk, but mainly as it made working easier.
Before you say "that's your opinion dude" - It is, yes, but I even OWN an analog summing setup. A nice one! The system I have at least has some op-amps and transformers in it which by their nature will introduce something harmonically, but some of the passive systems are literally wire and resistors. What you are doing there is adding another set of D/A then A/D conversions to your precious audio. How does that make it more analog? It's LESS analog! And THAT process alone changes your tone, as does the need to them make the the gain with some kind of preamp.
So here's the big question: Sonically, is it $6000 better? Hell no. I probably SAVED more money than I made from it by not leaving on on 24/7 like I used to. (I cut my energy bill in half, truthfully) It really is about setting up your tracks properly. Although you will read people talking about the near infinite headroom in a DAW, if everything is slammed to the top of the meters some things won't work as well as they should and having headroom in 24bit audio isn't going to hurt you - you can always make it louder later and it will teach you to build a track better. Back to the idea of summing. It's not a bad concept, or practise, but the idea that you HAVE to have sixteen outs and a system to sum pisses me off. There is value to the idea though. In my hybrid setup, I could choose to insert analog gear over stereo groups, and I always had some kit sitting on my stereo bus. I didn't even change the bus compressor threshold usually, as I always made sure I hit it in the same place level wise. That's the magic of using some VU's on your outs. You know where you are when it comes to analog land!
You can even emulate a summing setup. Why not make some subgroups and bus to these instead (it's good practise anyway as it makes making stems and alternative versions way easier) and try one of the many available analog channel emulations on these groups, and you will find a palette of far more flexible colours and tones that might just give you a little something that these summing boxes might be bringing to the party. Or not! Slate do a load of things suitable for this, such as the VCC, or VTC collection, so do Waves with the Redd, or NLS products, or perhaps try the Black Box HG-2. Another really nice tool is the Kush Omega series, which is the mic amp/tranny tone software part of their preamp system, much like the passive systems you make up with mic pre gain. As engineers, we're always looking to gain the edge, and as newer engineers we want to believe in the idea that we may just be missing some magic that will transform our lives...if only I had NS10's, summing, or a U87... but seriously don't always believe the hype. We have incredible tools now, often they aren't the old ones and it's the best time EVER in many ways to be making records.