DL-103 + A23 SUT + Pro-Ject Tube Box DS = ?

I will say it again:

This is the worst tonearm for Denon DL-103 low compliance cartridge. 
I have no idea why people brainwashed so hard, but Denon is NOT good for modern turntables/tonearms, because it was designed in the era where HIGH MASS tonearms were normal (it was designed in the late 60s). Today nearly all tonearms are mid mass, but Denon compliance is extremely low even for mid mass tonearms. Do you what does it mean ?  ** Here is the article. *** 

If you want to use your Denon in a proper way you need a super heavy tonearm, something like FR-64s, but definitely NOT a Pioneer tonearm. 

Your Pioneer is great for vintage MM/MI cartridges, they are superior to Denon 103 in everything and you don't need such expensive SUT at all (you can buy better cartridge instead). 

Why in 2020 you need a cartridge from 1969 with Conical tip ?
Can you explain ? Who told you this is the right choice for Pioneer ? 

It's so easy to search for a proper tonearm for Denon 103 and SPU, they are all high mass tonearms by default. 

Hi!
First of all, thank you for your magnificently forward response. I need a beer now, preferably two.

This cart is loved and recommended by many, even stock version is Stereophile Class C recommended component (this month’s issue, just showed up yesterday), where it sits in the company of Dynavector DV 10X5 and Ortofon 2M Black, both of which are lovely carts and cost close to three times the price of DL-103.

I have a modified DL-103 (by Zu Audio, MKII, mine is Grade 2), and it is a Stereophile Recommended Class B component. Feel free to glance:
https://www.stereophile.com/content/recommended-components-2019-edition-turntables-tonearms-cartridg...

Scroll down about 2/3 of the way. While you are there, look at Class C recommended turntables, and you will find that PLX-1000 was recommended, in the company of VPI Scout Jr, at twice the price, if we were to add $100 for Ortofon 2M Red to the cost of Pioneer, which comes stock without a cartridge. I am currently using Ortofon 2M Red with Pro-Ject Tube Box DS phono stage and it sounds great, but not even close to DL-103 I’ve auditioned a couple of years ago (read on...)
*** Takes a break for a minute, opens second beer ***

Zu Audio themselves recommend Denon DJ VL12 Prime as a turntable of choice, and I probably will upgrade with time, waiting for them to release some extra tweaks for it. That turntable stock is certainly in the same class as base Technics 1200 series as well as Pioneer PLX-1000, tonearm details are not included in specs, so I don’t know which turntable has heavier/lighter tonearm. What I do know is that cart/turntable combo sounded magnificent in Zu Audio’s room/booth at RMAF a couple of years ago. They offer a custom counterweight for their modified cartridge because stock one weighs 8.5 grams and modified one weighs 14 grams.

https://www.zuaudio.com/turntable/groovetracer-counterweight-zudenon-custom
Interestingly they refer in comments about that counterweight to Rega tonearms, and Stereophile’s Art Dudley [RIP] used Rega RB300 as one of the tonearms in his review; he loved Mk.II

https://www.stereophile.com/content/listening-184-zu-dl-103-mkii-phono-cartridge

as well as the original
https://www.stereophile.com/phonocartridges/1207zu/index.html
So, any thoughts on my original question - what settings should I use on the phono stage? Anyone else?

Thanks!

Most SUT's are expecting to see 47K ohm loading at the phono preamp so I would go with that. As far as capacitance I don't think it's near as important as a MM cart so set it at about 200pf and see how it sounds.That A23 is VERY nice, my buddy uses one for his DL-103 cart on his Garrard 401. Sounds very good into his Shindo phono preamp.
I'm using a Denon AU-320 for my DL-103 and am happy with that as well but if the right deal came along on a A23...........
BillWojo

Reading reviews is interesting, but reviewers all have their own value rating system, which makes for a difficult comparison.

Is the least expensive Class A as good or better then the most expensive Class B? Unless you have heard it in you system, who the heck knows.

Quoting reviews as if (A=B+C)/3=B is how things work. It just is not that way. Component matching and synergy is what really matters.

The Denon cartridges and Rega tonarms are both praised as great values. That does not mean they are really good, just good value.

The Pioneer turntable is OK. The ProJect phono stage is OK. The Denon cartridge is good for the money. Not sure how the works out in the ABC rating world.

Set your phono stage for standard MM phono settings

Thanks everyone!
For what it's worth, I by no means am saying that just because two components are "placed in the same class" by Stereophile (or anyone reviewing anything for that matter) that they are equal.  IMHO reviewing is subjective by nature, we are all humans and opinions are part of who we are, part of our individual personalities.  What I am saying, however, is that I expect two components in the same class to perform comparably, not one being vastly superior to another, regardless of price.  Ortofon 2M Blue is placed in the same class as 2M Black by Stereophile.  Do they perform the same?  I don't know, I don't have a $1K to spare to find out.  I would expect not, given that 2M Black cost three times more than 2M Blue.  I have a friend who own a 2M Blue and loves it.  He is about to upgrade to 2M Bronze and will loan me the 2M Blue to try out in my rig.  Then I will be able to directly compare 2M Red to 2M Blue in my rig, on my terms.  And I will not say anything to myself or others about 2M Black because - guess what? - I have not heard it!
@jperry  - Thanks!  Synergy indeed is what really matters.

Happy listening!