Is Mark Levinson still considered up there with the best of the best?

From my personal experience  any new amp will be challenged  to keep up with a well serviced pair of 33h mono blocks. 

I have the opportunity to listen to used ML 333 and ML536 in comparison to used Krell FPB, Pass Lab X350, X600, and new generation amp

Class D amps .

For the price point , used ML 333 and 536 are still very good options and sounds better than any Class D amps 

What matters is the designer, not the manufacturer or the promoter/salesman/marketer. The designer is responsible for the sound ... the marketer just sells it. If a manufacturer changes their house sound, that means the design staff has changed or moved on to greener pastures. This turnover is pretty common in the hifi biz because the actual hands-on designers are often poorly paid and their identity is hidden behind a corporate screen of Non-Disclosure-Agreements (NDA’s). By the way, never agree to an NDA in the hifi biz, especially if it is informal or based on a handshake agreement. That’s lawsuit territory. Don’t go there.

It’s fairly easy at a hifi show to know if you’re speaking to an actual engineer or designer versus a marketer. A marketer/salesman will hide behind a screen of vague buzzwords, and will get angry and dismissive if pressed hard enough. When I was wearing my Tech Editor hat, I was thrown out of exhibit rooms more than once by marketers at trade shows. By contrast, a real engineer or designer will be quite open and friendly, and will let you know if your questions are getting too close to NDA trade-secret territory.

I’ve done both marketing and designing, and they are very different things. Marketers are often technology-averse, and fall back on impressive-sounding buzzwords that don’t have any specific meaning. If they invoke Quantum anything, yes, you are dealing with a marketer and should leave quickly if you value your sanity and your wallet.

@lynn_olson  The designers don't always get to build the amp exactly the way they want to, when working for a manufacturer. I've heard Nelson pass say that when designing an amplifier for other companies they changed this and that to meet budget.

The power relationship between designer, corporate CEO, and the marketing department are different for every company. Quite a few well-known high-end companies (not going to name names here) are little more than marketing departments, hiring designers on a as-needed basis with no engineering continuity between projects. The engineering staff is basically a revolving door, resulting in products that might sound good, but not much follow-through as older products are phased out and the original designers are long gone. When you see a lot of "churn" from year to year in the product line-up, that’s what’s happening behind the scenes.

If the marketing staff are the only element of continuity in the company, the products may physically look the same from year to year, but what’s inside can be quite different. This is especially true if parts of production are moved offshore, taking QC and service support with them.

If the founder, or spokesperson, don’t understand how their products work, there will be a superficial continuity, but the new-hire designers will have trouble understanding the previous products on a deeper level. The schematic doesn’t usually tell the whole story, and the previous designers may ... or may not ... have documented the design philosophy of what they were doing. We’re not talking about Apple-sized teams of scores or hundreds of engineers here; no, more like one, two or three people per audio company.

This is why I said in the previous post to pay attention to the designer, not the name of the company. If the company is the size of Apple or Microsoft, there is a continuity of engineering culture over the decades. This is not true of audio companies, where engineering "teams" are often no more than one to three people, so if they leave, basically nothing is left.