As a fellow VA owner, welcome. Your Haydn Grands are very similar to my Bach Grands, 4 ohm speakers and just a little less sensitive. As such, a good dose of high current clean power will get them singing the way they should. So I would definitely recommend a separate 2 channel amp be dedicated to driving them using the L/R preouts on your Marantz. This will free up your Marantz AVR for other speaker duties too, if you have them.
I drive my Bachs with a Parasound 5125 power amp. It is a 5 channel model putting out 125 wpc into 8 ohm, 185 wpc in 4 ohm. It will drive 5 speakers to just about any volume I want without breaking a sweat. Parasound makes a 2 channel model, the 2125, with identical specs for someone only wanting a separate amp for the front two channels, like in your case. I would recommend this model to you without hesitation. Around $775 new; less if you shop smart. Since we do not know your budget, Parasound also makes their HALO line of premium but higher priced amps.
The above specs are typical of what you would want for your Haydns. It doesn't have to be Parasound. Emotiva puts out a great line of value priced amps with impressive specs to get the job done with Viennas. Again, just stay around no less than 100 wpc CONTINUOUS, both channels driven power into 8 ohm. Even though VA's are 4 Ohm speakers, just about all amps are rated into 8 ohms first, that is why I am using the 100 wpc into 8 ohm as a benchmark. The 4 ohm figure will usual follow and always be higher. With separate power amp, the wpc rating will usually be CONTINUOUS, unlike the confusing ratings of a lot of AVR's. In other words, you can assured of proper power when using a separate power amp.
Besides Parasound and Emotiva, there are many other brands that will get the job done too. Brands like NAD and Rotel. Other names escape me at the moment. Finally, the technology of a good Class A/B power amp has changed little over the years. Even a good, well kept used amp will work too. So check out some of the used models being offered on Ebay, audiogon.com and usaudiomart.com. This will also familarize you with all the other models available. Good luck in your search.