#1
My band has a problem: we don't know if we should stick with being an instrumental band or get a singer. We already have some material and it's very good but me, the drummer and our manager think that it's too dull after a while. So we thought about adding a singer to our band. But if we stick to the instrumental band, what should we do to sound less dull?
#2
Maybe you can experiment with new sounds, different arrangements, just mess around with ideas.

What style do you play?
Guitar: Fender Standard Tele

Amp: Marshall JCM900 4100

Pedals: Boss DS-1, Dunlop Crybaby, Dancelectro CC-1, Ibanez LU-20
#3
We play psychedelic/blues/hard rock/stoner. But yeah, we mess around with some riffs but yet, it feels very poor. It's mainly the lead guitarist just soloing like there was no tomorrow.
And we experiment new sounds with pedals and feedback, etc.
#4
I'm in the same situation. We started instrumental and then decided that we needed a vocalist as we didn't think our music was good enough to hold an audience without lyrics.

Then of course there are long passages in our music that don't seem to need vocals. So if we just got a vocalist they would be somewhat left out for large portions of the songs we already have written.

We could just rearrange the songs to fit a full time vocalist but instead we have just decided that we are going to do the vocals ourselves as and when we think they are needed.

If you stick to not having a vocalist I would advise listening to some instrumental bands to see what other musicians have done to keep the music interesting. So a lot of classical music, some jazz (for example Miles Davies, John Coltrane) and also post rock bands.

Try:

65daysofstatic
Explosions In The Sky
Godspeed You! Black Emperor
This Will Destroy You
Russian Circles
Mogwai
Pelican
Red Sparowes
Animals As Leaders

Even if these don't match the style you play you'll be able to take ideas and make them your own (which I guess you should be doing anyway!)
#5
Instrumental bands are generally only popular among other musicians. Until recently, how many high schoolers (other than guitar players) had any idea that Joe Satriani even existed? So, if you're okay with that, stick with it. If you want to change (as a band), then change. Its your music, do what you want to with it. If someone doesn't like what you're doing, screw them, they can go listen to Lady Gaga or whatever bullcrap studio project some corporation is vomiting out this month.

#6
The thing with an instrumental band is that you have to be incredibly proficient on your instruments, otherwise no-one will give a sh!t. Unless you guys are talented enough to bring in crowds on your own, a singer is pretty much essential.
#8
get a keyboardist who could also run a laptop with different loops and such... saw a instrumental band last night in your same genre who did that and they were outstanding. it just adds so much more you could do musically
#9
Being in an instrumental band myself, I'd have to say it all really depends on what you want
I personally know our music would sound awful with vocals and ruin the whole idea

With that said, there are more inherent probelms in being in an instrumental band as far as promoting yourself goes
Some people can't wrap their heads around it

But if you make your music to impress other people I think you're missing the point of making music to begin with

So the morale of the story: Follow your dreams!
Yes.