Separate Phono Pre - $$ before I will hear a SIGNIFICANT difference vs. integrated?

Without knowing anything about your particular NAD, what I have heard from all receivers anywhere and at any price point, they are crap compared to what you can get for the same money in any integrated. Its simple math and economics. Any time you try and do so many different things in one box you are forced to use only the absolute cheapest parts. No surprise, like everything else in life, you get what you pay for. 

So yes any outboard phono stage is just about guaranteed to be a very big and easily noticeable improvement. It will also open up a whole new range of upgrades. Because whatever phono stage you get can be made to sound a whole lot better with the right power cord, interconnect, and fuse. 

With a budget of only $150 however how likely is it you'll be dropping another $300-500 on wire and fuses any time soon?

A much more cost effective upgrade would be to go from the reciever to an integrated amp with a good built-in phono stage. This is your best shot at what you want, a SIGNIFICANT step up. 

I’m NOT a big believer in "exotic" Wire and Fuses. I AM a FIRM believer in solid, quality connections.

My Unbalanced Interconnects (ALL current interconnects) are presently Mogami Bulk types and Speaker Cables are #12AWG Belden.

If I go with the PS Audio Stellar Gain Cell (Line Stage with decent built-in DACT), I may add their new Stellar Phono Amp later. [It sounds like they "sweated the details" for a good product at a less-than-insane price.]

My guess is that in the meantime - something like the Stellar Gain Cell with a Schiit Mani will probably STILL sound better than what I am currently using. [For CERTAIN with Line-Level / Digital Sources with room to further improve Phono source later if I so choose.]

Or, I could go with the Emotiva XSP-1, which has a "Decent" built-in phono stage that could be abandoned and upgrated later with a Separate for that part.

For ~$375 USD you're not going to do much better than this phono stage if you're set on getting a separate.  No affiliation.

https://www.usaudiomart.com/details/649528359-tom-evans-microgroove-wpsu/