Louisl, basically what will serve you well is an amplifier that doubles its power output as impedance is halved, i.e.100wpc into 8 ohms and 200wpc into 4 ohms, etc.
The reason for this is that some speakers, such as your VR4 Gen III HSE have low impedances in the bass frequencies relative to the rest of the frequency spectrum. If an amp does not double power output into lower impedances (bass frequencies), then the result is less volume in the bass relative to the mids and highs. We often perceive this imbalance as brightness because the speakers are not producing the bass frequencies that would balance the highs.
For more in depth info, I suggest Robert Harley's "The Complete Guide to High End Audio".
Having said this, the Moscode 401HR, which I know matches well with the VR4 Gen III HSE, does not double its power as impedance is halved...yet it works really well with the speaker.
BTW, you can also run into impedance mismatches between preamp and amps. A preamp with high output impedance matched with an amp with low input impedance can result in rolled off bass (and rolled off highs) as well.
