Looking at the FR, impedance and phase angle curves of the Silver 200, it is not that hard to drive but they can benefit a lot from a subwoofer for sure.
If you can feel the sound "open up" at volume above 50, assuming at 50 you are already happy, then you will not hear much of a difference with an external power amp, especially the PMA-1600NE that has about the same output as you AVR, 2 Ch driven (you said stereo).
Volume at 50 is almost 30 dB below reference, assuming after running Audyssey the level trims are set to +/- a couple dB from 0, your X3500H is just cruising well below the speed limit!!
Can you check the level trim settings and tell us what the left, right center channels are set to? Then we can tell you approximately how hard you are actually pushing the AVR, at volume 50, or 60. I bet not hard at all, and that at 50, the output might be less than 0.1 W average, up to may be 10 to 50 W maximum peak. That's just a guess, can be firmed as soon as I know your trim level setting, or if you know the SPL you are listening to at your listening position and the distance.
Keep in mind, volume setting means very little as depending on gains, speaker sensitivity and distance, it can change with different power amps. For example, the internal amps of the X3500H is about 29 dB, so if you use it with an amp that has gain of 32, such as some older Emotiva amps, you will be able to turn the volume down a few clicks even if the Emotiva amp is less powerful than the AVR. Conversely, if you pair the AVR with a 500 W Hpex amp that has about 26 dB gain, you will have to turn the volume up a few click, all else being equal, that includes listening to the same sound pressure level.
Based on the specs and measurements, Denon AVRs and the PMA amps should have no sound signature, you may think they do if you are convinced by the marketing hypes. At distortions level that low, frequency response that flat, and the FFT (that shows the harmonic profile) that clean, you will hear a difference between the internal amps and an external power amp with similar specs/measurements if you expect the difference because of the marketing information and price tags, but that won't work if you don't know which amp you are listening to. No harm trying though, and it would likely work well as long as you don't do any DBT to find the truth (sarcasm not intended).
SoundStageNetwork.com | SoundStage.com - NRC Measurements: Monitor Audio Silver 200 Loudspeakers
See that the bass dropped rapidly from 80 Hz down, though depending on placement, they can sound great even with a subwoofer, especially if you use the Editor App tor run Audyssey and do some customization after. That's how you can get even better sound quality without spending money on an external amp. The App only cost $20 and Ratbuddsye, the 3rd party UI that allows you to customize easily, is a free download.
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