they are stock compatible in EF95 mode
They are very spacious and detailed with good dynamics... VS the 6AK5 family of tubes its a big step up... I had the Mullard 8083 at one point and greatly preferred 6HM5, 6DT6, 6AV6 heck even the EH90.
The point being is the traditional tubes these amps were designed around may be good... but there are far better... start with the post I linked above as a reference and check out all the tube reviews myself and others have done throughout this thread. I'd limit your searches to the 6HM5, 6DT6 and 6AV6.