You're already happy with the Rotel's power so you may not need 500W>8ohms.
The reason I recommended 500W is because the power needed to play music loudly, like symphony finales and also dynamic spikes from flutes and bass drums, doubles for every additional 3dB of SPL. If you want to play symphonies with 105dB peaks to make you cry at the end, then a 300W>4ohm rated amp will be well into it's power bandwidth, and you will start to hear it strain because distortion rises very quickly as you approach the full power rating at 1% distortion. If the rating is bullshit, like only one channel or exaggerated then you will clip when you try to pull 105dB. This rule of doubling of power for every 3dB of SPL is for pure resistive load, which does not exist in a dynamic speaker, and so does not account for impedance dips, negative phase angle or compression from the speaker. A small 84db speaker will be very compressed at 105, which means you'll want even more volume to compensate. If the amp is only 200W>8ohms you will be near the rated max power when you get to the end of Shostakovich Symphony 5, and it will feel tight and anxious like the system is struggling. It is not an inspiring sound so you will turn it down and miss the punch line of the whole symphony. You want to be able to turn it up! In addition to the power headroom, a more powerful amplifier has the additional benefit of sounding bigger and more confident even at low volume because the microdynamic contrast is increased because of the bigger power supply reserves that are audible at low levels.
Speakers with very low sensitivity are usually dynamically challenged compared to high sensitivity speakers when played at the same volume. They are often lower distortion, too, so that is the tradeoff. So the more powerful amplifier helps to loosen the speaker up with it's bigger current reserves stored in the power transformer's magnetic field and the power supplies larger smoothing cap bank. It can manhandle the lazy speaker to straighten up and fly right. I like high sensitivity speakers for this reason. They operate at very low current most of the time, with no compression at all until over 100dB. So when the 105 peak comes, everything still sounds relaxed and smooth, and you hear the music, not the equipment. But there are very few high sensitivity AND low distortion speakers in the market.
McCormack and Classe are great recommendations for classical listener, as is Moscode. These are 3 of my favorite SS brands. But it will cost you more than $300 to get that level of power and refinement. Your speakers are certainly able to deliver all of that refinement to your ears. To get all the potential of your speakers, you need a high end amplifier, not a midfi amplifier because there is more to music playback than just volume level vs clipping. The Adcom is a midfi amp, so it will never be as refined even at low volume as a McCormack or Pass. Refinement is the tone quality that makes you notice how beautiful and realistic the music sounds beyond melody beat and words, that is what high end sound and audiophilism is all about. But an old Adcom will be reliable and makes the needed power if your are tolerant of the sound at high volumes.
Classical music is not compressed, so it will occasionally provide full scale signals when your volume control is turned way up to hear the quiet parts, like in Beethoven Symphonies. When the amp and speaker can handle it, it is a great feeling. When they can't, it's a knife stab. The alternative is adjusting the volume level during the song to accommodate the amp, or lowering the noise floor of your listening room to play at quieter volumes. Since you are committed to the Vandys, I suggest that you invest in a high quality amplifier to get the most enjoyment from them.
