There are a few fairly active threads that include information about the BT355 over at another forum. But for the sake of saving you some time searching I'm going to point out a few observations;
1. The BT355 is a good solution for those needing to use a Line Out Converter from their head unit to their AMP. There are other options, but considering the size, relative expense and user experience with the BT355 it is a solid choice.
2. When you receive your BT355 LOC be sure to check the factory adjustments for gain and HPF. At least one user received a new BT355 and one of the three adjustments was way off. The BT355's have marks for the default factory set location and those should always be your starting point.
3. The RCA output voltage as delivered from BT appears to be very conservative. Based on experiences in the other forum, it is safe to increase the gain setting a bit on both channels without introducing distortion, but you will get higher volume from your speakers. When adjusting the BT355 gain, some of us have maxed the radio volume (17 clicks) and set the output voltage from the BT355 to 2.0V. That seems to be a pretty good compromise. On my BT355 that corresponds to somewhere between 1 and 2 o'clock on the settings dial.
4. If you are pairing an AMP with adjustable gains, it would be a good idea to reset your AMP gains after messing with the gain control on the BT355.
5. If you are using a 1kHz test tone to set AMP gains with a multimeter (based on voltage) or an oscilloscope (based on SINE wave) the BT355 may get a bit warm or even hot depending on how long you are pushing the 1kHz test tone through it. According to BT this shouldn't be an issue and I can confirm that in at least two instances that it wasn't anything to worry about. Now I wouldn't recommend doing this for hours on end, but it doesn't take long to get the gains set correctly
6. Most users have left the frequency settings alone, but they are there if you want to experiment.
7. If your AMP has adjustable frequency settings i.e. HPF, AP, LP the standard recommendation is to set the AMP to AP (sometimes referred to as "Full" or "All Pass")