I too am going soft and am getting some quotes for this. First chap I've spoken to seems to making a bit of a thing about fitting sound filters. Are these really required?
I think it's a silencer that's built in to the exhaust. It's not so much for internal noise as for external . Walk down a pontoon on a cold night and you can always tell the boats that have an Eberspacher.
We've had Eberspacher on all our boats - it's not too noisy inside, especially on the lower settings, but the exhaust does roar a bit on a still night. Havn't heard of it, but perhaps there's a silencer available now.
I have a Mikuni, basically the same as the Eberspacher and there are 2 silencer, one on the inlet and one on the exhaust. Started it first time without the inlet silencer and would not have believed the noise reduction when the silencer was fitted if I hadn't heard it myself! It made a huge difference, go for it.
If it is, it's well worth having. On previous boats, I've just had an ordinary Eberspacher installation (DIY fitted). My current boat has a factory-fitted Eberspacher with a silencer in the hot air duct immediately after the heater. It makes a massive difference to the noise level inside the boat. You literally don't know that the thing is running. If you can get one of these, go for it!