It is nearly a month that I listen SM64 through the Jr and I really enjoy the pairing.
I have never felt the need to add an amp to drive the SM64 that are known to be quite demanding, for IEMs (specs tells 98ohms). I listen at 35-37, sometime 40 on certain records. That is more than enough for me. I gave a charge to my Cayin C5 last evening to give it a try only because people asked me to do the test. On first attempt, after a week or so with Jr, I didn't notice anything. Let us see after a month.
What I noticed after a while is that Jr positions you right in front of the stage. First row, may be on stage for certain records.
I love the pair in the fact it gives details with a bit of warmth that make listening fun. Not as dark as FiiO DAP, sure. Just warm enough for me. With more low and high than X1 on line-out. It is usual that I find myself tapping my foot in office. Hum, embarassing (but so fun).
I retried C5 with Jr this morning and to my hearing there is no difference in SQ on the SM64. But C5 is clearly another beast in term of power. It took me a moment to get same amount of volume on both configuration. C5 with Jr maxed on line-out is at 2.5 on 10 while Jr must be at 40-42 on 75 (pretty high for me). At first I thought soundstage was much larger on C5 but in fact volume makes a lot into that. Once volume at same level the difference gets more subtile. C5 seems to reveal a soundstage a little larger on certain records that make it easier to separate instruments. More relaxing.
So Jr is, for my hearing, a good pairing with SM64 (I like these IEMs but I know it is not the cup of tea of everybody) and its slim form-factor is a real win. C5 will definitively not commute or travel anymore. I will keep it for listening in the couch, just hoping it will not take too much dust
time will tell.
It is just my little experience. I listened to MP3 through Hifiman RE0 on phones a year ago before converting to FLAC on the X1 through SM64 (and C5 to drive them). Then Jr now for a month. So take my opinion for what it is, just personal. I advice you find a shop where you can test Jr and make your own opinion with the SM64 (and if possible your music).