1. Devices with RCA output sockets: These have a nominal level (0 VU) of -10 dBV (316 mV) and an output impedance of a few hundred ohms. In order to supply the weak signal to a load with an 4 dBu (1.23 V) XLR input, the level must be raised by 12 dB. This can be done, for example, with the Sonifex RB-UL2 device. Its XLR output can drive inputs with impedances greater than or equal to 10 kOhm (current XLR line level standard) but also ancient 600 Ohm inputs. Don't try to solve the problem with a step-up transformer (there are such products on the market); they do not work satisfactorily because their output impedance is far too high.
2. Devices with an XLR output (0 VU = 4 dBu) that are to feed a device with an RCA input (0VU = -10 dBV): A 12 dB reduction is required here. You can achieve the best quality with a Jensen PC-2XR. Your Altec 15095A transformers do basically the same thing, i.e. they lower the level by approx. 12 dB if you connect the high-impedance side (pins 1/8) with the 4 dBu source and the low-impedance side (pins 1/6) with the -10 dBV RCA input.
3. I have no personal experience with devices with instrument level output (-20 dBu). Others know more about that.
4. Regarding your question about the Sony and Opamp Labs transformers, you'd have to know the exact type to figure out what they're capable of. If they are externally small, they are probably not suitable for the distortion-free transmission of low-frequency levels that we are talking about here.
5. And: The Altec 15335A was a good input transformer in its time. However, it is not comparable to a modern top product such as the Jensen JT-11P. The latter has very high noise rejection and transmits much higher levels without distortion.
