Hi Dani,
Thanks for starting this thread, should be interesting.
Seven spirits are mentioned later on in Revelation too:
Rev 3:1 ...'These things says He who has the seven Spirits of God and the seven stars'
Rev 4:5 'And from the throne proceeded lightnings, thunderings, and voices. Seven lamps of fire [were] burning before the throne, which are the seven Spirits of God. '
Rev 5:6... And I looked, and behold, in the midst of the throne and of the four living creatures, and in the midst of the elders, stood a Lamb as though it had been slain, having seven horns and seven eyes, which are the seven Spirits of God sent out into all the earth.'
There is similar imagary in Zechariah 4
These seem to refer to the seven spirits as God - which would be consistent with the context in the greeting:
Grace and peace to you from him who is, and who was, and who is to come, and from the seven spirits before his throne, 5 and from Jesus Christ
one interpretation I've come across is that the seven spirits refers to 7 aspects of the Holy Spirit as mentioned in Isaiah 11:2
'The Spirit of the LORD shall rest upon Him, The Spirit of wisdom and understanding, The Spirit of counsel and might, The Spirit of knowledge and of the fear of the LORD.'