Based on the info provided, you have the DM 603 S2, the specs are:
Sensitivity: 90 dB, 283V, at 1 meter
Impedance: 8 ohms nominal, 4.3 ohm minimum
Power handling: 150 W into 8 ohm unclipped
As shown in the calculated results, the A870 seems adequate assuming in an apartment environment you won't be listening to louder than 75 dB average and 95 dB peak.
If you would like to have a healthy headroom, then adding a 200 WPC power amp will get you at least 3 dB (or 2X) more than your actual need.
The Monolith 200WX3 are probably the best in terms of $/Watt, but not good in terms of lbs/watt. So my recommendation would be either the MCA 325 if your budget allows, or 3X Outlaw M2200 as Verdinut suggested. The M2200's list price was $399 but often obtainable for $299, or buy two get one free when sale. That probably won't happen now, not until the virus thing is done I guess.
Since you are open to a new AVR, I would say just get an AVR-X3600H that has just been measured and the results are really top notched relative to Yamaha's equivalent (sort of only as there are no directly comparable models between the two). In fact, if used as preamp the Denon beats Marantz top model and also beat quite a few separates that cost much more. You shouldn't pay too much attention to those measurements for sound quality issues though because its mostly academic, only "golden ears" can tell them apart (but they may also fall apart) anyway regardless of the measurements, linked below. Still, all else being equal, good measurements could enhance the Placebo effects and/or expectation bias that are often real to many people.
AVR-
X3600H ASR review,
you may also read more about why I (and others) recommended this AVR and its little brother the AVR-X3500H so many times on this forum.
The AVR-X3600H
This thing has impressive measured outputs under different conditions, such as:
Into 4 ohm, two channel driven: 152 W @ 0.0056% THD+N, or 184 W @ 1% THD+N
Into 4 ohm, 4 channel driven: 107 W (THD+N increased to about 0.018%), see post#272 in the ASR review thread.
If you grab this AVR, you can just sit back and enjoy without worry about an ext amp until your power requirement changes. Do invest on a $12 fan to enhance longevity as it will run warm.
The good thing is, the 603 S2 should be much easier to drive than the original 603 and slightly easier to drive than the newer 603 S3 based on the info on the B&W website as well as the available measurements for the 603 S3 by Stereophile.
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