Your FUN is probably defective, so feel free to RMA it. My FUN is perfectly compatible with my 120dB/V IEM's.
However, none of the above amps were designed for IEM's.
I don't have FUN. But i had CV2 for very long time. According to Burson's website, Fun has similar SNR as CV2. CV2 is very noisy compared to o2 atom etc. According to calculation of specs from burson it has 283uV noise. What a joke. No doubt it's very noisy. Even attenuated it's still too high.
I have FUN, and did experienced a strange noise issue with the headphone output. I raised the issue to Burson and they were not sure what was the problem but acknowledge that the noisy headphone output issue was indeed a problem (not normal working condition).
I found this issue during op-amp rolling, and the issue started when I plugged Burson V5i op-amp. FUN does exhibit higher noise with V5i than with other op-amps, up to the level that the level of noise is not tolerable. I was using IEMs for the op-amp rolling. So it seems the issue was with the V5i. I sent the V5i yesterday to Burson for investigation and replacement.
Using other op-amps, testing with my most sensitive IEM, the 1964 Ears V3 universal (Sensitivity: 119 dB spl/mW at 1kHz ) old discontinued model, there is some moderate level of hissing noise that for me is still 'tolerable' considering the high power output of the Burson FUN. Anyway I won't use FUN for such sensitive IEM, simply doesn't make sense. So in my opinion, considering the 2 watt output, that level of noise on a 119 dB spl/mW IEM, FUN can be considered low noise. Again in the context of a 2 Watt output amp. Using other less sensitive IEMs like DUNU DK-3001, the hissing noise is very low and didn't bother me at all.
I did SNR measurement of the headphone output using QuantAsylum QA401, and here is the result:
Headphone Ouput SNR on 33 ohms load (lowest measurement selected) @ 1kHz - 1Vrms (0 dBV) input:
At 2 Vrms (6 dBV) : 97.2 dBA
At 1 Vrms (0 dBV) : 96.8 dBA
At 0.5 Vrms (-6 dBV) : 94.8 dBA
At 100 mVrms (-20 dBV) : 85.6 dBA
At 50 mVrms (-26 dBV) : 79.8 dBA
I probably not very sensitive to hissing noise, for me, based on my own experience with SNR measurement using QA401:
SNR greater than 85 dBA: perceived as totally quiet.
Between 80 to 85 dBA: mild hissing noise might be audible.
Less than 80 dB: mild to moderate audible hissing noise.
So for a 2W amplifier with 79.8 dBA at 50 mV output, in my book it is pretty good. I guess the THX AAA 789 would be a lot better.
Sorry OOT in this THX AAA 789 thread.