Hi all, i have put this post together as i couldn't find the answers out there and so had to figure it out myself. So i hope this information might be of help to someone in the future.

I have a Raspberry PI with the latest v7 PiCore + 3.5" touchscreen + Jivelite + IR receiver. All works nicely, but i wanted to use it with my NAD 320BEE amplifier. Now the problem is, this amplifier (as many do) enters a power-down state after mains power is applied and I have an Alexa-controlled socket powering both the Pi and the amp, so i can turn them off eg overnight. I didn't then want to have to dig the amp remote out just to turn the amp on to fully powered mode every time i use it.

Incidentally, the AMP has an 'IR-in' socket and I can confirm that the 3.3v gpio which drives the IR LED will also work when directly plugged in - the signal in the socket is expected to be modulated exactly the same as that which supplies the LED (I checked on the NAD schematic) and the 3.3v range is sufficient to control the amp.

LIRC testing
It took me all of 5 minutes to test this with an IR-LED i had connected to GPIO-5 and using LIRC with this config file: http://lirc.sourceforge.net/remotes/nad/SR6
# this config file was automatically generated
# using lirc-0.8.3(default) on Wed Nov 26 13:31:18 2008
# contributed by Amigac
# brand: NAD # model no. of remote control: SR6

begin remote
name NAD_SR6
bits 16
eps 30
aeps 100
header 9038 4332
one 675 1556
zero 675 454
ptrail 662
repeat 9064 2113
pre_data_bits 16
pre_data 0xE13E
gap 107932
toggle_bit_mask 0x0
begin codes
KEY_POWER 0xA45B # Was: on
off 0x13EC
IR-tools and ir-ctl
However, when i installed Jivelite and set up the kernel IR-tools - no dice!
Long story short; after some input from Ralphy and others, it seems LIRC and IR-tools are mutually exclusive. So i did a lot of digging and discovered that within the set of 'IR-tools' there is a utility called ir-ctl:
ir-ctl is described quite aptly as "a swiss-knife tool to handle raw IR and to set lirc options" - http://manpages.ubuntu.com/manpages/.../ir-ctl.1.html

Note : I had already set gpio-5 as the lirc output in the tweaks - IR section even though i was not using lirc - i think you need to do this so as to add the following line to 'config.txt' : dtoverlay=gpio-ir-tx,gpio_pin=05

With no additional extensions or add-ons you can just try it out on the command line: ir-ctl -f should give you this:
Receive features /dev/lirc0:
- Device cannot receive
Send features /dev/lirc0:
- Device can send raw IR
- IR scancode encoder
- Set carrier
- Set duty cycle

I guess the receive function is already occupied (by jivelite) - but transmit is all ready to go. There are a range of options, but rather than create a custom binary file, we want to keep it simple and i had already verified the above keycodes from the LIRC conf file. Fortunately, i had a scope to hand and so i was able (spent several hours trying) to figure out which protocol and sequence the NAD amp uses.

Note to all; the NEC and NEC extended protocols (which were relevant to me) do seem to be very popular, but there are many others too (all supported by ir-ctl). It is the protocol which defines all the mark-space ratio, delay between header and command etc etc - this is the purpose of the first section of the lirc.conf file shown above. However, all i wanted was the Manufacturer's code (aka pre-data above) - 0xE13E and the specific keycode for power on - 0xA45B !

Just in case this helps you out - here is the full plot from my scope from the reference remote (NAD SR5):
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The carrier frequency for both NEC protocols is 38KHz, which is the default anyway for IR-tools and is generally most popular. Here is some more info: https://www.sbprojects.net/knowledge/ir/nec.php

After the low / high header, there are 4 x 8 bit fields: corresponding to the 16 bit manufacture's address and an 8 bit keycode followed by 8 bits binary inverse of the keycode for robustness -> that means NEC extended (NECX)
Alternatively, if the manufacturer's address were only 8 bits, then we would have 8 bit manufacturer code + 8 bits binary inverted manufacturer code + 8 bit keycode followed by 8 bits binary inverse of the keycode -> that would mean NEC regular (NEC)

From this and the description of the NEC Extended protocol (16 bit manufacturer code) - i realised that although the 'on-air' sequence yields: E13E A45B, in order to get this you need to send (via ir-ctl) lsb first for each byte, so the required hex code is 0x877C25. In order to use key scan codes, we need to use option -S, which requires the argument 'PROTOCOL:SCANCODE' as follows:
sudo ir-ctl -S necx:0x877C25

Each of the bytes above, if converted to binary and swapped lsb to msb will then match the bytes below !

This results in the following bit sequence from the IR LED:
11100001 | 00111110 | 10100100 | 01011011
E1 | 3E | A4 | 5B
Addr Lo | Addr Hi | Cmd | Cmd inverted

Last step then is to add the above command to one of the User command entries on the tweaks page of your piCore web gui

I now have a fully working AMP control solution right from piCoreplayer and, in fact i could send any of the remote codes i want (although i will have to figure out how to add some user commands to the squeezlite menu - e.g. switch input on the AMP for radio ?!

As i say, I hope this info is helpful for someone out there !