These; Trifid, Swan, or the Lagoon nebulae, and the Crab nebula, are very bright DSOs and are easily visible from dark skies with no filters in an 8". Bellow are my recommendations for better DSO viewing.
Steps to take for better DSO viewing.
1) Dark skies. The darker the better. In Bortle 2-3 I can find 40 or so galaxies in Virgo with 10" dob. In Bortle 1 skies it is closer to 70 galaxies. More and more objects become visible in dark skies. View DSOs well after the sun has set and when the moon is not up (near the new moon)
2) Dark adaptation. Our eyes have two types of sensors. Rods and cones. The cones are our colour vision sensors and the rods are our night vision sensors. While the cones are active, a chemical is produced which inactivates the rods. Thus, when looking at bright coloured light, our night vision sensors are shut down. It takes up to 30 minutes for the chemical to dissipate from the retina. Thus, not looking at bright lights for more half an hour will make faint DSOs far more visible. Using a red gel for a phone or laptop when looking at software will also help greatly. Dark adaptation is more important than most beginners think.
3) Averted vision. As noted above, our eyes have cones and rods. The cones we use for normal daytime vision are located in the centre of our retina. The rods are located on the periphery of our retina. Because of the peripheral location of the rods we can't actually see faint DSOs if we look directly at them. We have to kind of look slightly above or below the DSO to see it. This is called averted vision. You might have noted that the DSOs are more visible when scanning back and forth with telescope or binoculars but when you stop to look at the DSO it kind of disappears. This is because when you are scanning the light is falling on the rod cells at the periphery of the retina but when you try to focus on the DSO the light is not falling on the sensitive rod cells but on the less light sensitive cone cells. Averted vision takes practice. I could not see any detail in galaxies when I started out, now with practicing averted vision and properly dark adapting, I can see the spiral arms in many brighter galaxies.
4) Filters (Discussed above). Emission and Planetary nebulae can be more easily seen with filters. Specifically UHC filters really improve many nebulae (Oiii and H(a) filters also help but on fewer objects). UHC filters allow the nebulae light through but not any other light increasing the contrast of the nebula (the nebula is not bright but the background is darker). Objects like the Veil nebula, North America nebula, rosette nebula, helix nebula (all very very large DSOs) become visible with a UHC where they are difficult to detect without a UHC.
5) Aperture. The bigger the telescope aperture the more light that is gathered and generally the easier it is to see the DSOs. I can find bright DSOs in my 90mm telescope, most of the Messier objects are visible from dark skies but beyond the really bright DSOs the 90mm does perform well. I can easily find hundreds and hundreds of DSOs in the 10" dob. Larger and larger apertures open up more and more objects.
6) Magnification. For galaxies and emission nebula, start with low magnification, this will make the light from the DSOs concentrated in a smaller area that is more easily identified by our dark adapted eyes. M33, the Triangulum galaxy or M101 (both spiral galaxies) are great examples of this effect, they are much harder to detect at higher magnifications but become more obvious at lower magnifications. Once detected increase the magnification to see if the view improves.
9) Practice, Practice, Practice. I had a difficult time finding DSOs when I first started out. Now many that I thought were difficult to find, I find obvious so much so that they seem very bright. Where an object was just a blob it now has detail. A great example is the whirlpool galaxy. I thought it was just a fuzzy blob when I started, now I can see the spiral arms and it looks like the main galaxy is stealing stars from its companion. I had a really difficult time finding M110 and M32 near Andromeda, now they are just plain obvious to me. It sounds funny but we need to learn how to see DSOs (seeing is something we don't normally think of as taking practice).