Hi mmporsche,
As several have noted above, auditioning and selecting an analog front end is probably the most difficult purchase in audio given the number of interdependent variables, and thus I wish you good luck in your search. I think Inna puts forth some pretty good recommendations in his earlier post on price ranges, although I might allocate more budget to the cartridge on a percentage basis. With that in mind, I offer some recommendations below at the price points that I find to represent the law of "significant diminishing returns," as referenced in your post, to provide some context on those proffered price ranges. (Please note that if you consider used turntables / tonearms / phono stages, this would have a significant impact on the prices at which diminishing returns kick in).
Starting with the turntable, the SME 20/3 is a good option - I have auditioned a few tables (and currently own one) costing 2-4x as much, but none of those tables were what I would call "significantly" better - the only tables I have heard that were that were so retail for $50k plus. I definitely had my share of Memorex, "blown away" moments with the SME 20/2 (Solti Ring on Decca vinyl), and still remember it fondly. And, it matches your requirement of a "set and forget" kind of table - the SMEs are no fuss and built to a high standard - I have referred to them in the past as the CD players of vinyl. Finally, given the comparatively low value of the British pound, it presents a pretty good value right now.
Re: tonearms, the $4-5 range, where a number of long time stalwarts reside, seems to me to be the point of diminishing returns - e.g., the Graham Phantom, the Triplanar, and to a lesser extent, the SME V. If you have a little more to spend on a tonearm, the Kuzma 4-point is a good option as well.
In my view, cartridges tend to be significantly dependent on one's personal preferences, and there are such a variety of options it is hard to propound recommendations without more of a sense of for what you are looking and what tonearm you would ultimately select. For example, depending on what you value in an audio system and your tonearm, the diminutive Denon DL-103D may be all you need. I note your dealer recommended a Benz Micro LPS, which is a pretty good cartridge, and I don't think you need to spend too much more to reach the point of diminishing returns. Other options in that price range with which I have had good experiences include the Dynavector XV-1S and the Ortofon A95; the Clearaudio da Vinci v2 may also be a good option, but I have less experience with it. If you are looking for a more budget-friendly option, the Shelter 90X is the best comparative value in cartridges I have found.
For phono stages, it is a bit more challenging to make a recommendation in terms of "significant diminishing returns," as I have not found one clear price point where the level of improvement drops off as price increases. Taking for example some of the phono stages referenced above, the Herron is a very good candidate at around $3600, but I found the Lamm and ASR Basis Exclusive to be notably better (at 2-2.5x the price). However, I would note I don't have as much experience in that $5-8k "sweet spot," so there may be some lesser priced options that would work for you. Ultimately, given the comparative ease of demoing phono stages as opposed to the rest of the analog front end, I would tend to agree with folks above that you stick with the Parasound for the moment, and once you have settled on the rest of the front end, explore your phono stage options.
Overall, I think at the price point you referenced above ($25k), you can get a phono system that you gets you within shouting distance of the best that is out there, which you can enjoy for a lifetime without any thoughts of upgrading. If you go used (for everything other than the cartridge, which I recommend you buy new from a dealer), you can get there for $15k or less. Again, best of luck.
