Practically, too high level limits the dial range of the preamp’s volume control.
Set ADI-2 DAC’s Reference Level to -5 dBu and Volume to 0 dB.
This is on the safe side for signal quality and nice to handle.

If you want to go into details:

Unfortunately, the specs published by Musical Fidelity in M8s pre’s manual are a bit strange and hard to believe:

Input sensitivity 0.352 V
Overload margin 12 dB

Calculating based on this, the highest input the XLRs can take would be 1.4 V or +5 dBu.
A typical standard CD-player puts out 2 V / +8 dBu, which should overload  M8s’ input ?!
I don’t think so.

Best, check with a full scale 400 Hz sine wave, from any source e.g.:
https://soundcloud.com/liv-zanfardino/4 … -frequency

Or an online sine wave Generator, set 400 Hz, and volume to 100%:
https://www.szynalski.com/tone-generator/

Bring down M8s’ volume dial close to minimum, but not off.
Play the sine wave through ADI-2 DAC and slowly turn volume and Ref Levels up until audible distortion kicks in.
The transition is clearly audible.

This is the max. level M8s can take.
Now use any level not higher than this one.

Reference Level +1 dBu, Volume 0 dB would improve SNR from 120 to 123 dB.
This is not audible anyway, but ….

If you run through this, I’d be curious to know your results

The ones that write the manual might not have checked with the tech department - or the figures simply are true.
The specs for the M6S sound more reasonable, 12 dB above 4 V equals 15.9 V

It‘s really easy to check:

For convenience, switch: I/O / Line Out / Settings / Auto Ref Level: “On”
Now you can control ADI-2’s volume continuously across it’s full range without interruption.

Play 400 Hz / 100% from the linked generator.

Listen at med/low listening level (preamp volume dial) while turning up ADI-2’s VOLUME dial continuously, until the sound starts to become different, kind of bright or scratchy.
It’s obvious, you will clearly recognize the transition point for sure.

Ignore the little momentary glitches when ADI-2’s relays switch the reference level, these are normal.

Remark:
With Auto ref Level “On” Volume will be shown in –dBr instead of straight dB.
dBr references to ADI-2’s maximum output of +19 dBu on XLR.

With:
SETUP / Options / Display / Hor. Meter: “Post-FX dBu”
you can directly read the output’s dBu (add 6 dBu for the XLR I think).

BTW: nothing can break during the process.

As suspected, this means the specs in the manual are wrong.

You’re quasi free to select any Ref Level you find convenient.

Reference Level +1 dBu and Volume 0 dB maxes ADI-2 DAC’s SNR / Signal to Noise Ratio, while the level is still low enough that the pre’s volume should preserve some dial range.
To gain more dial range - 5 dBu / 0 dB sacrifices 3 dB SNR - still magnitudes ahead of any audible noise - if you don’t hear noise there is no noise problem.

DAC’s resolution - much more important than SNR - is the same for all reference levels.

As mentioned, choose any level you like.

Approximate by ear to unify the loudness.
ADI-2’s volume works in steps of 0.5 dB, so a perfect equality might not be possible.

MF M6x’s XLR’s 4V equals 14.26 dBu.
Setting ADI-2 DAC to nominal +7 dBu (= XLR +13dBu) / Volume +1.5 dB would be the closest you can get.

@ketalar

I am very sorry for the late reply, but I missed it here.

Yes, I have only one source (Audio PC). I did a long time of testing before buying the MF M8s PRE preamplifier to drive the 2×MF M8S-700M monoblocks, and finally bought the MF M8s PRE preamplifier.

This setup PC->RME (DAC) ->Preamp MF ->Power amps MF has significantly higher sound quality than PC->RME (preamp+DAC) ->Power amps MF.

I wanted to save costs and not buy the preamp MF and stick with the RME itself and use it in preamp+DAC mode. However, the difference in sound quality is very significant even though I did some blind tests. I can't explain it technically, but it's just the way it is.

Could someone please explain why this is so?

Thank you

I have no explanation, I am a simple music lover, not an engineer ; but :
The MF M8s PRE looks superb !
Can you tell us  :
- What do you exactly mean by "significantly higher sound quality" ?
- The analog level outputs of the RME -> Amps and of the preamp ?
- More about your loudspeakers (+listening room), and about your musical tastes ?
Regards,

@ketalar

Thanks for the response, I'll take the liberty of replying:

Ad What do you exactly mean by "significantly higher sound quality" ?

As I said - the combination of monoblocks with MF preamp sounded significantly better - I can't describe it, but it was most evident during quiet night listening, the sound was absolutely crystal clear with MF, instruments very separated, and incredible dynamics. The same behaviour at high and very low volumes. When I plugged in the RME preamp it was also excellent, but the details were not as sharp, and especially at low volume the system was less resolving and it was "veiled" or I don't know how to describe it more accurately. I tried it endlessly and then I just had to buy an MF preamp, as the combination of monoblocks + MF preamp is fascinating. I don't know what it is and can't explain it as I'm not an audio or electrical engineer, just a music lover and audiophile.

Ad The analog level outputs of the RME -> Amps and of the preamp ?

AUDIO PC and EUPHONY AUDIO SYSTEM see https://euphony-audio.com/v4/

Ad - More about your loudspeakers (+listening room), and about your musical tastes ?
Loudspeakers: Fyne audio f502sp see https://www.fyneaudio.com/product/f502sp/
Room: approx. 23 m2
Music: all genres - classical, art and hard rock, metal, jazz....  I probably enjoy the monoblocks + MF preamp the most during symphonic classical music, the dynamics and separation of the instruments is just superb. But all genres sound fantastic. The power, authority and control speaker of the MF  combo is incredible. It's addictive and takes listening to music to a whole other dimension.

No consumer preamp exist changes the dynamics, the difference between quieter and louder parts of the music.

Even tube amps only can saturate if over-driven VERY hot, but this causes clearly audible distortions.

For dynamics reduction a recording studio device called “compressor” is needed, but this has other, partly negative side-effects like the so called “pumping”.