I own a Reference 6SE and have listened to the LS28SE. A friend of mine bought one as well, so I have listened to them in more than one setting.
It is an outstanding preamp and very similar to the Reference 6SE. Same sound qualities, detailed, musical, great midrange bloom. The Reference is just a little bit better, bit lower noise floor, a bit more dynamic… cut from the same cloth. It is a question of finances. If you have a choice and can afford the extra money, get the REF 6SE or REF 10. If not, the LS28SE is outstanding.
The LS28SE throws a great wide and deep soundstage. But I have not had one in my system to directly compare with the REF. I detected no hardening of the high frequencies in the LS.