How to get rid of interference inside (and around) the system unit?

Question

Audio

I decided to start measuring the parameters of sound devices, I bought ASUS Essence STX specially - the characteristics are excellent, but now there is a problem. The line input catches everything that is not laziness. Right down to interference from the TV in the next room (by the way, is it interference over the network or over the air?)
The card itself seems to be shielded, but this does not save much. Actually, a few questions:
1. What connecting wire should I use to connect line-out-> line-in? Those. I want to buy two mini-jacks and the cable itself, so the question is which one is better to take? To not catch everything like an antenna. Now I have 1.5 meters of the factory for 40 rubles. - leaves much to be desired. And yes, there is also the question of interpenetration of channels. You need a stereo cable with good channel separation so that there is no mutual induction.
2. How to get rid of interference in general? Those. everything climbs to the input - both 50 Hz and 15.5k from the TV - how to exclude at least what climbs from the network? I was told about an isolating transformer 220 V (galvanic isolation), but it’s completely incomprehensible - how will it eliminate interference from the network if they all jump to the secondary winding anyway?
3. Well, perhaps the main problem is earth loops between the measuring and measured cards. Yes, such that the card in loop-back mode gives better results than when connected to Essence STX. What to do with it? Will an isolating transformer help?
My case is grounded (the earth is my own, very good, separate from zero). I tried to disconnect from the ground - the measurement results improved by an average of 3 dB. But at the same time, a lot of interference appeared for ASUS in the loop-back mode. Those. it turns out the effect of the earth loop has decreased, but the pickups have become stronger.
In general, my brain boils. Who has dealt with similar things - I ask for your help.

Answer 1

According to clause 3, the RF will not fit through an isolation transformer, the characteristics of the core are not the same, but where do you get it, and how much will it cost - XZ.
Are the measuring card and the measured one in the same case? are you really that confident in the ground?
I deal with things like this from time to time. I had problems with pickups only when measuring the frequency response of the amplifier> 100dB.