Sound Impressions of Restoration/Upgrade
Before I started the restoration/upgrade, I listened to the Marantz 1090 for a couple of days. I even took a few notes of these initial listening impressions since I didn't want to rely on just my sonic memory (especially after a period of >1 month). The 1090, even in stock form, was pleasant to listen to, it does have the Marantz "house" sound: somewhat "warm" sounding with a full bass. It didn't sound as smooth as the early-to-mid 70s Marantzes that I have heard (e.g., the 2270 receiver or the capacitor-coupled 1060 integrated amp); it was more like the later 70s Marantzes (e.g., 1152DC, 170DC). I did make a note that the treble range was a bit "brash" sounding; nothing too obvious, but noticeable at higher sound levels and with recordings with lots of percussion or brass instruments.
In the ~4 weeks since the final upgrades to the 1090, its been played for a couple of hours most evenings. IMO, the restoration/upgrade has changed the sound for the better (did anyone expect otherwise?). The differences in sound are not "night and day", but are readily noticeable with attentive listening. Previous to the restoration/upgrade, as mentioned, the treble range was a bit "brash", but now the treble range is very natural sounding with lots of detail (e.g., the different cymbal sounds are easily discerned).
The sound of the bass range (<300Hz) has also changed, but not as much as was noticed in the treble range. The bass is now subjectively deeper and "tighter" sounding. I think that what is often referred to as "bass transients" is also better, but this may be attributed to an improvement in the reproduction of the middle frequencies. Indeed, reproduction of vocals does seem very clear now, all of the little vocal "ticks" and 'hiccups" are easily discerned and hard to decipher "mumbled" lyrics can now be understood.
An unexpected improvement was in the noise "floor"; there was a very slight background noise/hiss that was easily heard with the volume control turn up past 11 o'colck, but that is now completely absent, even with the volume set to maximum. At typical listening levels, the music seems to emerge from a "blacker" background silence. Playing CD and LP recordings that I'm very familiar with (e.g., Steely Dan/Aja; Miles Davis/Kind of Blue; Dire Straits/Love Over Gold; Beatles/Abbey Road; Fournier/Bach Solo Cello Suites; Duke Ellington Orchestra/Digital Duke), I was hearing the little details that are in the background much more clearly.
I have noticed that many on the Marantz forum have an admiration for the lower powered units. I do not think that is misplaced, the lower powered units often have a simpler signal path and this contributes to their good sound. I think that this is one reason that tube topologies often sound quite good, they have much simpler signal paths. What the lower models often lacked is the use of the best quality parts, e.g., whereas the higher and TOTL Marantz units would have film capacitors the lower units would have to make do with electrolytics or even ceramics. The sound of any unit is the result of the circuit topology and the implementation of the topology. The sound of a good vintage design can be much improved by upgrading with better parts that weren't available (or were deemed too costly by the bean counters) when the unit was originally manufactured. Conversely, a poor design will continue to sound poor even with the most costly capacitor upgrades. The 1090 is definitely in the former group; its design is quite advanced (e.g., fully discrete transistor design with a high capacity power supply; a DC-coupled main amp with class A driver stage; practical tone controls that can be switched out). Admittedly the build quality of the 1090 isn't as robust as the earlier series (and this cannot be changed) but its sound with the restoration/upgrade is without doubt, top tier.
