Phono Stage, How much do I need to spend, New or used.

You have $650 cartridge and $125 phono stage now.
I think you can fond some nice phono stages within $1200 and you don’t gave to pay more than this. 

One inexpensive phono stage I like a lot is JLTi mk4 which you could find for $700, now mk5 available directly from Australia.

You want to fill the room "like digital does"? Impossible. They all sound way better than digital. However, if you are asking what phono stage will be so good that even a guy like me who uses digital as the standard will be forced to admit yeah, this is WAY BETTER? Herron VTPH2A.

Or if using the MM cart then you have enough output to use the Decware ZP3.

I must warn you however, either one of these and your days of comparing to digital are done for.

That Mani really is your bottleneck. I have a friend with a Well Tempered Amadeus and Denon DL-103 who was using one. He replaced it with a Pro-Ject Phono Box RS (1000.00) and he says the difference was astounding. I would recommend my K&K Audio Maxxed Out phono stage (~2500.00) but I believe it is no longer in production. I have owned this one for over 20 years and it has been updated a few times. It's one piece that will probably outlive me.

So yes, a new phono stage will be a huge improvement over what you have.

Oz

OP I thought I new it all as a kid (still do to some degree). I had this Mac MX110z. I'm going to tell you it had a really good phono section (so I thought). I had Heavy platter RCA with a Grace tonearm and cart.. It weighed 125 pounds and was a 3 piece work of handcrafted art from a local Masonic lodge makeover in the late 60s.

It had an separate phono section. That section was plugged into a Marantz Valve setup. My first experience with Marantz.. No one could afford MARANTZ.  Fisher, HS Scott, Mac, Heath, Dynco, etc..

Well that phono section was a Marantz/Ear (Masonic lodge) special..
All valves, point to point. with straight wire turret boards. It reminded me of HK Citation layout.. Just very well laid out..

I got the TT, 3 Grace tonearms, the phono section and at least 100 valve
for just moving it as as a kid. It was out, and the new was in... Good when the whole family is affiliated in some way or another..

Point being if I wouldn't have gone overseas, and new I was coming back. I would have NEVER sold that phono section.. EVER. I kept the Mac...  LOL fate..

A well matched cart and phono section.. Not just any cart and any phono section... Some pres are easy to change. CARTS aren't.. There is only so much leeway on a cart..

Enjoy