When you talk of stereos in Morgans you will always be subject to disparaging comments from a bunch of people who think there is something wrong with listening to music whilst in a Morgan. And let's not even mention aircon, whoops I just did. Enough to say that we have, and use both, in the Plus 4. In our rather crazy July tour of Sicily we put the hood up and the aircon on in the heat of the day
My experiences and comments are therefore based on the many long trips we have done in the Plus 4.
Firstly touring distances in Europe. Europe is not so big that 1,000 miles in a day is even possible. 600 miles on only high speed roads (Autostrada, etc) would be extremely difficult even in a fast sedan, let alone a 3 wheeler. If you want to actually see anything, meet people, eat well, and experience the life of Europe I would say you should plan on no more than 400km (240 miles), if you are using normal roads, or a maximum of 600km (360 miles) if you are using predominantly Autostrada type roads. Sure if you absolutely needed to do so you could go Berlin to Paris, 1,000km (600 miles) in a day if you had to but it would take you at least 10 hours of driving without a pause and you would be "well done" by the end of it. My advice is relax and enjoy, 10 countries in 10 days will get you the t'shirts but it will not be fun.
You don't mention when you are are planning to come but I would recommend that you avoid the Mediterranean countries in the very hot months of July and August. There is not much protection from the sun and the heat in an M3W
Now the stereo. We have music (usually MP3s) nearly all the time in the Plus 4 (throttle bodies and sports exhaust, so a bit noisy) up to about 80kph (50mph) you can hear it fine, it starts to get more difficult to hear as you approach 100kph (60mph). In other words on secondary and regional roads it is fine. At Autostrada speeds, 130kph (80mph), it can become almost impossible to hear depending on road and wind noise. I would not use any form of earphones as I believe not hearing sounds around me would be extremely dangerous.
It is difficult for me to guess from my very limited M3W experience but I could well imagine that hearing the stereo music system would become very difficult at speeds above 50mph.