Hi natureboy,
I understand your reservations about committing to a budget but I think you'll be glad you did since members can now offer more specific options knowing they'll be financially acceptable to you.
I'm going to start with some general advice rather than specific options:
Speakers ($1,500-2000 budget):
Before you decide on a specific Magnepan speaker model, I think you should do some homework by listening to them in person. Hopefully, there's a Magnepan dealer within a reasonable driving distance from your home. Magnepan's website offers a dealer locator tool to assist you at www,magnepan.com
You should at least be able to audition and compare the 1.7, 1.7i, 3.7 and 3.7i. Listening to the 2.5R may not be possible unless there's a seller of a used pair near you.
In 2004, I was in a similar situation. My aging Magnepan IIBs were starting to de-laminate and I was looking to update. I auditioned the 1.6QR($1,700/pr. then) at a local dealer and was trying to decide between these and a used pair of 2.7QR ($2,200 new in 1994 but seller asking for $1,200 used in 2004) from a seller about 250 miles away. I liked the 1.6 but thought they were a bit bright in the treble (possibly not completely broken in yet) and weak in the bass. I bought the 2.7qr for $1,000 without even hearing them based on owners' reports that the treble was smoother and sweeter than the 1.6 and the fact the 2.7 were 3-ways and had a large panel section.devoted exclusively to bass frequencies. I really wanted the 3.6R but they were way outside my budget at that time. In retrospect, I made the right choice but know I took a big risk and was a bit fortunate, too.
I think you 're likely to prefer the 3.7 or 3.7i to the 1.7 and 1.7i and will need to decide if the difference is worth the difference in price. From what I've read from owners and professional reviewers. however, the 1.7i is extremely good and may offer the most value for the price and be the biggest bargain. You need to decide for yourself and you could always plan an upgrade to the 3.7or 3.7i at a future time.
Amps ($1,00-2,000 budget):
The most important thing you need to determine is whether your VAC Avatar tubed integrated can properly drive Magnepans. I believe the only meaningful method for doing this is trying it and for you to judge for yourself.
I don't have any experience with tube amps driving Magnepans so I cannot give you much assistance but others who do have basically stated "maybe" and that more wattage may be required.
However, I do have substantial experience driving Magnepans with solid state amps of various types and various wattages. My advice is that high current amps (those that double their wattage going from 8 ohm to 4 ohm speaker loads) perform very well on Magnepans whether they're class A, A/B or D and that performance improves progressively as wattage increases.
The most impressive improvement I experienced on my Magnepans (2.7qr that are 87db efficient) was moving from a 400 watt/ch class A/B amp to 1,200 watt/ch class D mono-blocks. It resulted in attaining the best performance from my speakers I've heard thus far, especially in the bass, an effortless presentation overall and an obvious increase in dynamic range.
Another option would be to use your VAC Avatar as a tube pre-amp and pair it with a high current, or at least higher wattage, tube, class A, A/B or D amp of your choice. Or sell the Avatar and do the same with a tube pre-amp of your choice. I am very confident that this option will work no matter which Magnepan you decide to purchase. Based on my experience, I would strongly recommend a powerful class D amp would work best.
Speaker Mods ($500 budget):
If you wind up buying the 2.5R, 1.7i or 3.7i, the only mod I can even think of be Sound Anchor or Myestands for them to help make them more rigid. However, I view these as refinements that are not initially required and can be added later if at all.
Tim