I tweaked your left join to return the first top result, but I didn't have a chance to really test it out.
Select Top 1000000 tblAssets.AssetName,
tblAssets.Domain,
Cast(tblAssetCustom.Custom4 As DATETIME) As [Date Assigned],
tblAssetCustom.Serialnumber,
tblAssetCustom.Model,
tblAssets.AssetID,
tblAssets.Lastseen,
tblAssetCustom.Custom3 As Company,
tblAssetCustom.Custom1 As [Assigned Employee #],
T1.Displayname As [Assigned Employee],
tblAssetCustom.Custom2 As Assignment,
tblAssetCustom.Custom5 As Technician,
tblAssetCustom.Custom6 As PO,
tblAssetCustom.Lastchanged,
tblADusers.EmployeeNumber As [Last On Emp#],
tblADusers.Username As [Last On ID],
tblADusers.Displayname As [Last On Name]
From tblAssets
Inner Join tblAssetCustom On tblAssets.AssetID = tblAssetCustom.AssetID
Left Join tblADusers On tblAssets.Username = tblADusers.Username
Left Join (Select adu.EmployeeID,
Max(adu.Displayname) As Displayname
From tblADusers adu
Group By adu.EmployeeID) T1 On T1.EmployeeID = tblAssetCustom.Custom1
Where (tblAssetCustom.Custom3 Like 'NoCo' And tblAssetCustom.Custom1 <> '' And
tblAssetCustom.State = 1) Or
(tblAssetCustom.Custom2 <> '') Or
(tblAssetCustom.Custom5 <> '')
Order By tblAssets.Domain,
[Date Assigned] Desc