Magnepans without the true-ribbon tweeters, such as your 1.7is, do lack some detail in the highs. Doesn’t matter what amp you pair with them, they will always lack some amount of treble detail compared to good dynamic cone speakers. Even when paired with the most transparent amplification in existence they will lack some detail, relatively speaking.
It’s most likely that your recollection of those older Maggies is based on their open, effortless soundstaging and imaging. That trait is what tricks the uninitiated into believing they’re more detailed than they are in reality. Also, they are very sensitive to toe-angle and can sound very bright if not positioned correctly, which probably also accounts for some of the perceived detail.
Don’t get me wrong. I love my Maggies. I own the .7s and previously owned the 1.7is. They’re great speakers but only in the context of their strengths (all speakers under ~$20K/pair have strengths and weaknesses). Those strengths are open/effortless scale and imaging, and midrange nuance unsurpassed by nearly any cone driver. They do have excellent midrange detail, just not so much in the highs. The true ribbon 3.7is have greater detail but lack the dynamic balance and imaging precision of my .7s. The LRSs are allegedly more detailed than the .7s. The 1.7is I owned were sorely lacking in midbass impact. So it seems to me that different Maggie models have different deficiencies, or strengths rather.
Anyhow, don’t go chasing detail that will never come. If you want Magnepan-like openness in conjunction with great detail, get a high quality coaxial monitor speaker with a stout cabinet (LS 50 Meta for example) and use subs to augment the bass.
Invariably, you’ll encounter some who will claim that Maggies are as detailed as any speaker when powered properly. I’d question the experience level of those who postulate such. My bet is they haven’t heard a good beryllium dome or a RAAL tweeter. That or maybe they’ve suffered too much hearing damage over the years.