Tenants do negatively impact the price, there is just no getting around it. It happens in 2 ways:
1) Quality - In most cases, a property with tenants is not shown in its best light. In the same rationale that staging a property with furniture and artwork can enhance a property's sale value and speed of sale, the tenants will do the opposite.
2) Time - It is more difficult to allow showings with tenants. If you were living there, as the seller, you would (probably) be as acomodating (sic) as possible. You have a stake in the sale. The tenants do not have any real motivation to be helpful. It is harder to get buyers in the property and, thus, slow down the sale.
If your property is going to be marketed at a low price to attract investors, tenants are no problem. In fact, they will be a bonus to an investor. But you will need to sell at a lower amount if you want to attract these buyers.
I hope this helps.