Your thoughts about an upgrade

I can't comment specifically about your current setup vs. Quatro CT/M5-HPA combo as I've never heard the M5-HPA, but what you are considering is a major step up in every way (investment included).

Going from the VTL to a separate pre/amp combo and adding sub(s) with the Treos could also be a significant step up for less money.

If you love the Treo and have the scratch, go for it. The Quatro should tick the same boxes as the Treo with more bass.

If money is an object, you could get to a very similar place starting with the addition of a good sub to your current setup. Going to a pre/amp/crossover setup with a sub could get you even closer.

Since you mention bass then this one is very simple. Long before a risky speaker change a much better move would be to add a distributed bass array, aka Swarm subwoofer system.

The beauty of this is virtually any four subs you get will blend seamlessly with your existing system. You’ll have bass like you never dreamed- fast, articulate, smooth, deep, and dynamic.

There’s a number of different approaches, all of which will work. Easiest is to just buy the Audiokinesis Swarm from Duke. $3k plus shipping for genuine state of the art bass. Hard to beat. Next easiest, buy any four powered subwoofers. Whatever you like and can fit and afford. Hardest but perhaps best is what I did, DIY four subs powered with 2 Dayton amps. Less than $3k even with real Rosewood veneer and two amps.

All these options will be less cost, less risk, and more bass than a new pair of speakers. ANY speakers!

This is also the ONE way of addressing what you said: "I’ve never turned the volume on the current system above halfway. I think I’m not hearing as much bass as I would like."

The bass volume you hear, how balanced it is, is very much dependent on your listening volume. This is why so many amps used to have a Loudness control. Loudness turns the bass up so it sounds balanced at low volume. But you don’t have to listen at low volume to notice this. Bass can sound weak even at moderate to fairly loud levels. The beauty of using a distributed bass array is you are able to set the levels to what sounds best for you with the music and at the volume levels you want to listen at. Can’t do this with a new set of speakers. And even if you can find ones that do, still leaves you with the problem there are only two of them. Until you hear how much better four are you have no idea what you’re missing.

Great advice above. Or you could simply add a REL S3 SHO and dial it in so it's just audible (you don't want to overpower the room) and also be amazed for a lot less effort.

I am curious though about where your Rega P3 is located in a corner and if it will be subject to acoustic feedback.

John_g, I think you would be pleasantly surprised by adding one or two subwoofers. REL's are currently on sale in many places including https://rel.net/product-category/serie-ti/?utm_campaign=October%20Sale%203rd%20Reminder%20Consumers%...
If you can afford it go with millercarbon's suggestion.