Thanks @oberoniaomnia
I was glad that I was able to hear the difference between tubes but disappointed that I didn't find the pricier replacement ones to sound a lot better. I do not even remember which tube I tried and I don't know what is in there now but I think they are Electro-Harmonix 6922.
The LS-16 unfortunately has 4 tubes which would make rolling more expensive. The E-H 6922s are cheap.
The manual says the original tubes are 6922/E88CC tubes and should last 4000 hours. Since I was given this preamp after years of use I cannot even guess how many hours these have on them.