Best way to go to Sendai for a concert?

I want to go to a Concert in Sendai the 29th of August but I don't know which is the best way to go from Tokyo.
The concert's door open at 17.30h so I would like to arrive earlier and I think it would be about 20h when it ends.
So I would like to know the cheapest way to get there, and if it could be fast it would be good too.
I thought about using the Seishun 18 Kippu but I don't know if there's a train or bus that can get me there, apart from the Shinkansen which can't be used having the Seishun 18 Kippu.
We are 2 people, that's why I would like to use an economic way to get there.
It would be good too if we could arrive at tokyo before 1AM on the 30th because that day is when we return from Japan.
The reason why I don't think of JR Pass as the first option is because I don't know if I could get the tiquets so it would be a waste to buy JR pass and not using it for this.
Thank you for helping me~

Tokyo - Sendai is best by JR Shinkansen. One way trip with non-reserved seat is 10,080 yen. About 2 hours. If you go by local trains, it would take several changes, close to 7 hours, and cost around 6,000 yen.

For the way back, the same Shinkansen departing Sendai at 20:55 will get you to Tokyo at 22:56, and 21:26 - 23:04. Another one at 21:48 - 00:05.If you need to return to Tokyo that same night, Shinkansen would be about the only coice.

The cheapest way is the express bus(JR Bus Tohoku).
For example, Shinjuku-station south exit bus terminal(depart 9:00 a.m. operated by Sendai/Shinjuku 0001 or 10:50 a.m. operated by Sendai/Shinjuku 0003) for Sendau-station east exit terminal(0001 arrives at 14:30 p.m. or 0003 arrives at 16:20 p.m.) for 3,000 YEN per person, single.

On the way back, Sendai-station east exit terminal(departs 23:50 p.m. operated by Dream Masamune 0112) for Shinjuku-station south exit bus terminal(0112 arrives 5:30 a.m.) for 4,500 YEN per person, single.
Tickets are available at JR East offices(Midorino-madoguchi).