Hmm.... Not sure I agree with that. Marantz and others had separates before they had receivers. They made the tuner 8B, 9 etc. amps, 10B tuner and 7C pre-all tube separates. Don't think they made receivers until they went solid state, but I could be wrong. The double digits series like the 19, 18 etc. But they were still selling separates such as the 7T, 33, 3300 pre amps, 15, 16, 16B amps, 20, 120 tuners etc. Then they started selling the 22XX series receivers and those sold like hot cakes. But I believe they were cheaper than a full set of separates.
Generally separates were considered better, a step up from receivers and more expensive. All in one receivers were popular because of the price, convenience of having everything in one unit and performance was decent. But stuffing a tuner, amp and pre in one box has some drawbacks because of space limitations. Except for maybe the totl receivers.
The 2100 isn't totl, but it's not bad. A good place to start. You can always trade up later. I have a 2110. I think it has the same tuner as the 2100, but with the added oscilloscope. I had it refurbished and it works like it should and like new. As far as performance, it's not my best performing tuner, but it will tune in the stations that I listen to just fine and sounds very good to me. I'm not complaining. And the scope is just sooo cool!. It matches my 3250B pre, 170DC amp and 5030 tape deck. To upgrade my matching tuner, I'd have to buy a 2130 and those things are $$$$$$$$, so that's probably not going to happen unless one miraculously falls in my lap... If I want to listen to a highly sensitive, high performing dx'ing tuner, I get out my Kenwood KT-8007, Technics ST-9030, Nikko Gamma V, Sansui TU-717 or the king of all my tuners, the Sony XDRF1HD.
But that's just my opinion...
To learn a little about Marantz,
this may be of interest to you.