Sorry for a third reply, but your post sounds somewhat close to my experience.
I think you’d benefit more from a better receiver than adding inexpensive tubes to your current setup.
I would look for a receiver that has pre-outs and is also 4 ohm stable. I really think being 4 ohm stable is a solid indicator of a quality receiver. I have been impressed with my Integra as a home theater receiver and am now experiencing their great customer support. They have taking my receiver back for repairs at no cost to me and it’s well over double the advertised warranty period.
I think you’d benefit more from a better receiver than adding inexpensive tubes to your current setup.
I would look for a receiver that has pre-outs and is also 4 ohm stable. I really think being 4 ohm stable is a solid indicator of a quality receiver. I have been impressed with my Integra as a home theater receiver and am now experiencing their great customer support. They have taking my receiver back for repairs at no cost to me and it’s well over double the advertised warranty period.