Marantz 20B Mystery Solved

Now that I've actually printed out the 20B schematic, I can see there
are only 4 filters, not 8 as I had previously thought.

The 20B schematic from MarantzPhillips included both Model 20 and Model 20B
schematics for the IF. Each shows 4 filters. I thought they were both part of
the 20B. The 20B version has the tap for AGC return to the front end, and a FET
replacing the first bipolar, but is otherwise the same.

So the mystery I proposed is solved. The Marantz 20 and 20B were clearly
designed as an audiophile tuner, with 4 filters (not uncommon in such) for a
single wide bandwidth. I believe that the 20/20B may have a wider bandwidth
than the 10B. Actually there was discussion previously here that Marantz 10B
does not have sufficiently wide band for lowest possible stereo distortion. 
Also, IIRC, the 10B has 6 filters, I need to go back to check my 10B schematic.

I don't know how this compares with other tuners, but each filter module in the
20/20B IF seems to have 8 reactive elements. So that would make 32 reactive
elements (poles?) in all.

WRT the 20/20B issue, I now admit that is an interesting question. It is quite
possible that either of these things in the 20B could have a slightly
detrimental effect:

1) RF amplification (could add noise and distortion)
2) AGC circuit (operates like slow feedback)

I'm not sure how fast the AGC circuit operates. I think it would be interesting
to replace AGC circuit with a switch that would either use normal AGC operation,
or set some fixed levels of amplification. And, having done that, remove the RF
attenuator too.

I think in my case, though, I'm going to need the RF amplification. My 20B
arrived this morning so I look forward to unpacking it tonight.