Scope probes rate their field level attenuation based on the assumption that the input impedance of the scope is 1 meg ohm. Since your SDR has a much lower input impedance, a scope probe will provide much greater attenuation than its usual 10 or 100 times rating
If the input impedance of the SDR is indeed 3.3 k ohms then a 10x probe will attenuate voltage by ~2728 while a 100x probe will attenuate voltage by ~3090.
You want to avoid transformers on the SDR input if at all possible. The scope probe or equivalent resistance should provide sufficient attenuation and isolation. You can add a DC isolation capacitor if you wish - select a value that provide less than 100 ohms of impedance at the frequencies you are measuring.
If you do not need to measure the voltages in the circuit under test and there are not too many competing signals, often simply getting a probe near the point of measurement, without making contact, will provide sufficient amplitude to confirm the presence of the signal.
- Glenn W9IQ