crocosmia look terrific with grasses, as would some of the reblooming daylilies. Dahlias, although you would have to replant (probably) the tubers, would give a long season of bloom and finally, you could look at penstemons.

I do not care for daylilies with grasses.... foliage is too similar.

Echinacea is often short lived.

Have you concsidered Rudbeckia 'Magnus' or Shasta Daisy 'Becky'?

yarrow isn't floppy if you ring it about 4 inches up... the ring gets hidden by the foliage. 5' grasses? you'll want something substantial. a small shrub might be a better choice.

Monarda/bee balm has many red varieties in a range of heights. Be sure to get one with mildew resistance.

Salvia darcyi (Darcy's Mexican Sage) is a perennial hardy to zone 6

There are several selections of Salvia greggii which are zone 6 hardy and red such as Furman's red and

Salvia 'Maraschino' (Maraschino Cherry Sage) is a hybrid between Salvia grahamii and Salvia microphylla.

Salvia microphylla 'Hot Lips' is red and white

http://www.highcountrygardens.com and http://www.plantdelights.com have quite a few hardy red sages between them.

Look into Helenium and Heliopsis.

How about various alliums? Or torch lilies?

Both would make a statement

How about one of the heliopsis such as 'Ballerina' (yellow-gold) or some of the Rudbeckia hirta hybrids which can be various combinations of yellow, gold and dark burgundy. The latter might have to be replanted each spring, but flower for a long time and will reseed a fair bit. They are also fairly drought tolerant.
Since the grasses are warm season ones, you might consider some spring bulbs to give you interest while the grasses are waiting for warm weather to get going.

I would vote for Persicaria Firetail. Check it out.

I'm in year two so would need at least another year to establish parameters. The only drawback would be that it likes moist conditions. Mulch would help and maybe a double strip of driphose.

What about Hot Poker (Knifopha) plant?. They come in shades of red, orange & yellow and have a growth habit compatible with grasses. They are also drought tolerant and hardy in your zone.

Thanks everyone, for your suggestions. I finally am close to a decision. I'm thinking of Salvia 'Furman's Red', but would like to also use Shasta Daisy 'Becky' in a few spots. Most websites say that it's long blooming (June - Sept), but I've seen some reviews that say it's not. I really want something light colored to contrast with the red salvia, but it must be long blooming. Can anyone tell me what the true bloom time is? Also, if it's short, can anyone recommend a substitution that is long blooming (and similar size and structure)?
Thanks so much!

I keep a bloom schedule and my 'Becky' lasts 4-6 weeks. Early June to Mid/Late July. I'm in zone 4b, Minnesota.

I do not deadhead, cuz I want the seeds. But I suspect if you did deadhead, you might get a second flush.

I've tried other Shastas and their bloom times were shorter.

My own experience is that Shastas give me the best mid summer bloom length.

Could you do a shrub... like maybe a hydrangea? That could give you light colored, long blooms.