Yes I own it, spent endless hours of labour working it to perfection- will never sell it. You have identified the weakness of the stock unit. My unit has no weakness. Bass extends further down, soundstage is much deeper/wider and less upfront. Highs are in proportion to the midrange and bass. Refinement and front/back transparency are amazingly life like. Highly sensitive to tube rolling both the 6v and 12v position. FYI, the SLP-2002 can and does utilize the 6H30 tube to great effectiveness. The Sovtek 6H30 is on the "darker" side, while the NOS "DR" is much more open sounding. I prefer 6dj8/5814 combination. Again, if you plan on keeping the pre-amp (its cheap enough) I will instruct you on modding it. Otherwise, just sell it off and move on is my advise. You will never get the bass to where it should be nor will it exhibit a balanced frequency response in stock form.
Later....
Later....