How are you connecting the Lamm preamp -- via the direct in or one of the selectable inputs? I looked at the specs for the Ayon and they don't say what the input sensitivity is. It could be that it is a mismatch for the Lamm. If you are using direct in, try switching to one of the selectable inputs and then reduce the volume on the Ayre until the hiss goes away. This will mean that you'll need to turn the volume on the Lamm up higher.
If you are using the selectable inputs on the Ayon, it could mean that the preamp tubes are aging and need to be replaced, or once again, just a mismatch with the Lamm.