Do I needto use a +25db mic-boost/preamp for a SM57 to profile an amp, or just mic straight in to Return?

an advice from an over 50 years old recording engineer:

record your voice with an SM57.

listen carefully.

if you think it captures your voice perfectly, go ahead.

if you like the way your amp sounds,do you think this a good choice to capture the sound?

A 57 straight into return works fine no need for anything else in the chain unless you specifically want that tonal contribution.

Totally agree with Alan,

also consider that the Profiler automatically adjusts the level when profiling, so to boost the signal before it goes back to the Profile will not give You a Louder output profile.

Instead it is probable you'll see a message during Profiling Process that sounds like more or less ''Please reduce Your Level 'cause You're F* Clipping''

see also

page Levels on manual

Quote

Levels

Set “Return Level” using the corresponding soft knob so that the level of the reference amp roughly equals the level of the internal sound of the PROFILER. You can check level differences by switching back and forth between the current Rig and your reference amp. If the OUTPUT LED turns red when you play your reference amplifier, the level is too hot. You might notice audio clicks when you turn the “Return Level”; this is the analog RETURN input stage switching the level in the analog domain, to ensure the best signal-to-noise ratio. This feature allows you to capture any signal level, from the softest microphone to the hottest studio level output. Don’t worry too much about achieving a perfect volume match; during the profiling process, the “Return Level” will be fine-tuned automatically to match the unity level of the PROFILER.

Please remember to leave a few decibels of headroom in your microphone preamps and the rest of the signal chain - the test signals can get a bit louder than a regular guitar signal and might cause clipping if you don't. Be sure to watch your level meters while the PROFILING is in progress, just to be sure there are no overloads.

Press the soft button labeled “Next”.

this sounds strange:

Quote from BayouTexan

I have to raise my gain on my interface to about 80% to get an appropriate recording level.

Quote from Sollazzon

Instead it is probable you'll see a message during Profiling Process that sounds like more or less ''Please reduce Your Level 'cause You're F* Clipping''

??????

Those Kemper profilers can be very direct ?

Quote from BayouTexan

Turning up the gain knob for mics is pretty common from what I understand compared to guitar pickups. Maybe not so much if the interface cost $1000+ and have built-in mic pre-boost.

yes, turning up the gain knob on a desk or mic pre is common in order to maximise signal to noise ratios by providing a healthy signal right at the beginning of the chain. However, the level of gain required depends on the source level (guitar amps can be pretty loud), mic itself and the preamp being used. The Kemper clearly has some form of mic preamp inside and has an auto level setting feature that ensures you are in the right ballpark.

You only need to worry about adjusting levels if you are way outside the range which the KPA can manage. In this case you would be in the “you’re F@#£*ng clipping” range that Sollazzon referred to or in the “speak up, I can’t hear you when you mumble” range.

If your question refers to something like a FetHead or CloudLifter, keep 2 things in mind:

1. You only need this for low output microphones (e.g. Shure SM7B) in combination with low volume sources (e.g. spoken word). A guitar amp/cab would typically be profiled at considerable volume where you should be more worried about the maximum SPL a microphone can take without getting damaged, hehe.

2. The above mentioned devices require 48V phantom power which the Profiler doesn't provide.