USRP B200 connecting RX and TX with attenuators

[SDR77]

Hello, I own a USRP B200 and previously i was doing my tests with the Pluto SDR. I am using the USRP B200 through GNU Radio. I want to connect the TX and RX of the USRP B200 with attenuators. I tried 46 dB and 36 dB attenuators, but I can't hear the AM modulated broadcast at 127 MHz. When I set the USRP Source gain to 55 dB and the USRP Sink block gain to around 25 dB, I can hear my voice, but it sounds noisy and robotic. When I connect the same attenuator and the same flows to the ADALM-Pluto, there is no issue. What dB level of attenuators should I use for the USRP B200, and should I use an external power supply? If so, what should the specifications of the external power supply be?

In below outputs is the scenario with 36 db attenuators, 55 db Rx Gain, 25 db Tx gain, USB 3.0 connected for power source and data transmission. Voice sounds noisy and robotic. Thanks for your helps.