The RS1i, along with other Grados, have the reputation of being difficult to pair with. I own/owned 8 pairs of Grados, and auditioned the whole Grado product line, I would say the RA1 & RS1i combo are, by far, the most satisfying and musical pairing.
Many Nay-Sayers bashing the amp for generally two reasons: 1, the company use some cheap components to build the amp; 2, the amp has relatively inferior sound quality in terms of clarity, speed, etc. True, the amp is not the most detailed/spacious sounding amp in the market, even not quite as good as it's rivals in it's price range, but I think it fulfill the most crucial part of this hobby, the pleasure of listening to the music.
Paired with most other systems, the Grado headphones share similar sound signatures: boosted treble and bass (some lower priced models don't have bass boost). Most Grados model can be fatiguing if paired with wrong system. I've tried the RS1i with many amps that cost several times more than the RA1, but the sound were either too bright, or the overly punchy bass make my ear hurt after long term listening. The RS1i can actually sound pretty impressive with some of these amps, but none of the pairing makes me very engaged for there always some part of the sound distract me from enjoying the music.
At times, I was disappointed by John's tuning and even go so far as, thinking his vision of sound is too aggressive and too focused on particular genre.
But I still didn't give up hope on the Grados as they sound vivid compared to aother manufactures in the industry. I recently read an article on touring the Grado company saids that Grado tunes his phones in his own system, and the RS1/RA1 combo should reflect his true vision of sound. My faith in him finally paid off. Paired with RA1, the RS1i sound open, clear and smooth, there isn't noticeable boost throughout frequency, the overall sound is quite neutral yet not "boring", everything sound vivid and natural, nothing distracts me from enjoying the music. There are certainly trade-offs for being smooth, it sacrifice a bit of details of the sound, but I believe increase of the smoothness worth the loss of details as it is better for long term listening. The sound quality may not as impressive as some other pairings, but the the combo sounds very satisfying. The tonal balance reminds of the MS-pro, just smoother. The smoothness of the sound also make me related the combo with the HF2 and PS500. I guess Mr Grado tuned these two models this way as he's noticed that not many people using his amp with his headphones
.
As we enter this hobby longer, we've develop more sensitive hearing that helps us to judge which sound are well-tuned whereas which are not. The sound of RS1i and RA1 combo is definitely a sophisticated and engaging one to me. If you are after musical pleasure, the combo will not disappoint you.
To relate the two component with the topic, the RS1i is the bacon, it's nice but salty, the RA1 is the egg and it's plain, they taste alright by themselves, but if you put them together, they just work wonders.
Edit:
The combo will sound best with warm sounding DAC and cables.
The design is indeed obsolete and overpriced.
I do fully agree that all that matter is musical enjoyment. If you are not getting gear that maximizes the subjective concious experience of listening to music you have lost yourself dammit. Our subjective concious experience is all we have. That is however not in opposition to being critical and rational when looking at buying audio related stuff.