You don't state where you got the new processors from. If they weren't obtained directly from HP, they may not be compatible with your server, or even with each other.
Processors typically have a microcode version known as the stepping, and if that stepping is not known to the BIOS, the BIOS may prevent booting to protect against system instability and data corruption.
Also for dual processors, the stepping needs to match across the two processors, or the system won't boot, either.
If these are not HP processors, you may not be able to find the stepping of your two 3rd party sourced processors without first booting successfully and then using a tool like CPU-Z.
A Google search reveals these as potential problems for you, which I've listed below.
Now, as to what steppings are compatible, HP may not reveal that as they may expect their end users aren't supposed to need to know that, and end users should instead be relying on HP to provide compatible parts. If you are lucky perhaps the 2015 BIOS update release notes will tell you what processor steppings are compatible with it.
HP ProLiant Gen8 Troubleshooting Guide
http://h20564.www2.hpe.com/hpsc/doc/public/display?docId=c03230516
Page 20:
- Processor Not Started (Stepping Does Not Match)
- Description: If a processor has a stepping different than the bootstrap processor, the processor is not started, and this message is displayed.
- Processor Not Started (Unsupported Processor Stepping)
- Description: If a processor has an unsupported stepping, it is not started, and this message is displayed.
- Processor Not Supported (Unsupported Core Speed)
- Description: If a processor has a core speed that is incompatible with the other installed processors, the processor is not started, and this message is displayed.
Page 47:
- Unsupported Processor Detected, System will ONLY boot ROMPAQ Utility. System
Halted.
- Audible Beeps: 1 long, 1 short
- Possible Cause: Processor and/or processor stepping is not supported by the current system ROM.
- Action: Refer to the server documentation for supported processors. If a ROM version exists that supports the processor,
- Power down the server.
- Insert a Systems ROMPAQ diskette containing the latest ROM version.
- Boot the system to flash the system to the latest ROM version. Allow 15 minutes for the process to complete. Successful completion is indicated by a series of beeps of increasing pitch.
Page 48:
- WARNING - Mixed Stepping Processors were detected. System cannot proceed.
- Audible beeps: 1 long, 1 short
Page 58:
- 242 - Unsupported Processor Configuration Detected – Processors are installed in the incorrect order.
- System Halted!
- Audible beeps: Long beep
- Possible cause: Processors are installed in the incorrect order.
- Action: Install the processors sequentially starting with Processor 1.