Picking up on what others have said, and I hope adding to it, instead of driving yourself nuts trying to "equalize" your room, there is a fairly reasonable path to your goal, costwise, using digital equalizers. Several of these have been reviewed especially by Robert E Greene of TAS whose obsession with flat in-room response is similar to yours. You should be reading his articles, not Holt's. Equalizing in the digital domain ameliorates or avoids phase problems, too. You might be able to find one of the units that REG has reviewed in the Agon for sale listings, for a "reasonable" price.
Having said that, I can tell you that I listen to most all my records at one of two volume settings on my preamp, and they are only one click apart, i.e., about 9 o'clock and about 9:30 on the dial. (I have a very high gain LOMC-capable preamp.) But almost all LPs sound fine at 9 o'clock. So have I reached some sonic nirvana by doing that? I don't know, never thought about it in that vein, but I AM loving my phono playback system these days. My cdp is on the workbench for some upgrades, for the past 2 months, because I don't miss it.
Having said that, I can tell you that I listen to most all my records at one of two volume settings on my preamp, and they are only one click apart, i.e., about 9 o'clock and about 9:30 on the dial. (I have a very high gain LOMC-capable preamp.) But almost all LPs sound fine at 9 o'clock. So have I reached some sonic nirvana by doing that? I don't know, never thought about it in that vein, but I AM loving my phono playback system these days. My cdp is on the workbench for some upgrades, for the past 2 months, because I don't miss it.