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It's a simple difference in sensitivity of the speakers, The Quatros are 87db efficient and Kensingtons 93bd, so you need higher volume to attain the same loudness. On the subject of tubes 5 years is a decent long time you may want to look at retubing or better yet getting your tubes tested if you have a local dealer with a tube tester. Hope that helps!
2500 hours is not an excessive amount for the small signal tubes in most pre-amps (there are exceptions) but it's getting up there for power tubes. At that point, not a bad idea to buy a set, if you can't find someone to test them. If there is a big improvement, you have your answer. If not, you have a spare set for the inevitable day when one or more tubes goes bad.
The problem with tubes is they slowly degrade. I recently purchased new speakers and they didn't really sound like they did in the showroom..... At first I attributed it to break in , which I think in all honestly is more of your ears acclimating to a different sonic signature. But then I thought , how many hours are on these tubes? So I retubed and what a difference. Not that it sounded horrible , but it was lacking dynamics and sounded recessed. Things really opened up with new EL34s. I would estimate they had about 2500 or more hours. I listen a lot , music , tv, and movies. For NOS tubes , Brent Jesse is a good source. For current production I would suggest Jim Mc Shane for the simple reason that your amps use four tubes each. He is reputable and will send you 8 matched tubes . I have monos and I know , you dont really have to have 8 matched but I like to have a matched quad , two in each amp just so both amps are "the same" with regard to tubes.